Let's argue about BC's

[Kevin Thomas]

Oh, Bryan . . . so many more productive ways to spend a Monday. Tried to warn you, dude.

 

[Michael Eichele]

See bold below:

Focusing on Michael's G7 BC results with the 178 Amax:
.248 from a low velocity round (.308 Win I think) and
.290 from a 300 RUM

.248 is only +3% from my value of .240. This is essentially a match, considering that the error only results in ~10" difference in predicted trajectory at 1000 yards. This is about as accurate as a drop test can be expected to be given all the uncertainties mentioned (chrono error, range error, etc) as well as unaccounted for effects like vertical wind, and coreolis (which could be up to ~3" of vertical in this case).

Now the same design bullet is fired from a different, higher velocity rifle. It appears to produce a G7 BC of .290. Since the weight and caliber of the bullet is the same, the only difference affecting the BC could be drag. Going from .248 to .290 implies that the drag (form factor) reduced by 15%. That's a huge amount. That's the difference between a bullet profiled like the 178 Amax (which is a relatively high drag bullet), and something even sleeker than a 7mm 162 Amax (which is an exceptionally low drag bullet).

When groper insists that the G7 BC can't be that high (.290) it's the same as saying the drag of the 178 Amax can't be as low as a 7mm 162 Amax. And he's right. That's a 15% difference, and bullets don't magically fly with 15% less drag from one rifle to another. You don't ever have to pull a trigger to know that.

But the trigger was pulled and the results are the results.

I think Michael and the others know that the bullet isn't flying with such a different BC, but are still left wondering why their tests imply the drastic difference. This is the question I'd like to focus on. Because to focus on the question of why the BC really is 15% different between rifles is a waste of time; because it's not that different.

The only purpose in my statements of different rifles is how they relate to velocity differences, which is not a waste of time.

Michael provided information that describes a fairly detailed and well executed test. Some basic questions I have about the test are:
1) Wind conditions. Is there any terrain that would generate vertical wind over the range you're testing? We all know what the smallest amount of wind can do to horizontal displacement. It's just as bad in the vertical plane if it's there.

I practice in the wind and test and verify in still air. This is why my tests are done in the dead of winter. When things get really cold, the air hardly moves if at all (at least in my region). I can say for certain that wind was not a factor. I realize a crystal clear picture cannot be made with even average winds.

2) The test that produced a BC of .290; was it ever repeated? If so, how closely was the .290 repeated?

Yes. .290 was a value that worked flawlessly over the course of the barrel's use including some long range kills albiet the tests were done to 1000 where it was only fired to 800 after that with machine precision.

3) Are you accounting for the velocity drop between the muzzle and chrono? A 10 fps drop in 5 yards (from muzzle to chrono) correlates to ~4" difference in predicted drop at 1000 yards.

I use a 15' from muzzle set up for any reason the chrony is set up. 15' is factored in for velocity corrections. All of my 'muzzle' velocities are just that. Muzzle velocities. Not 15' from muzzle velocities. The exception is when the double chrony method is used. One will be set at 15', the other at 615' where I dont correct for the 15'. I look to the ballastic calculator and compare the numbers 200 yards apart where I am looking to the velocity at 15' and 615' on the calculator.

Michael and most everyone else is deriving BC's from drop tests. One of the most critical measurement instruments in that experiment is the scope, so I'd like to ask some questions about that.

I'll question the assumption that a scope that's calibrated (meaning click values measured) when mounted to one rifle has the same calibration factor when mounted to a different rifle. Here are my reasons:

1) I suspect that when a scope is mounted with different hardware (rings, bases, etc) and clamped in different places on the tube, that it can effect the amount of internal deflection to the 'mechanism' that moves the reticle in response to clicking the turret.
2) Let's say on rifle 'A', the wind zero for the scope is in the mechanical and optical center of the scope. You establish a calibration factor for this wind zero. Now mount the same scope on rifle 'B'. It happens that the wind zero for rifle 'B' is 10 MOA (for example) to the right. Now, when the plunger pushes on the ROUND erector tube which houses the reticle, it's not pushing directly on the bottom of the ROUND cylinder, but instead is pushing up on the edge of it. I think this might result in a different amount of reticle movement per click compared to the situation where the plunger was pushing directly on the center of the round tube.

The above two possibilities are only my speculation, grasping for possible causes as to why a scope's vertical adjustment calibration might not be the same when mounted to different rifles. Keep in mind I'm not an optics expert.

Same rings, same scope. Obviously the rails were different since one was a long action and one was a short action. Both (at the time) were picatinny 20MOA rails. Scope is calibrated to .2645" per click (.2526MOA). My personal software accounts for the correction. I use the same scope for tests because I have not calibrated any otehr scope. It's a pain and it is easier to switch the scope for tests and experiments.

Back to the BC's. I'm not here to stomp my foot and declare that anyone's wrong because they got different numbers than me and my numbers are accurate. In this case, we can throw my numbers away completely and focus on the fact that someone did testing that implied the same bullet had a BC that was different by 15% when fired from two different rifles. I think we all accept that that's not actually the case, but the question we all want answered is: how and why would a test imply that?

Could it be velocity effects? I don't think so, and here's why. Using my latest test results, I've compiled the following correlation between G7 BC and velocity for the 178 Amax. Note that the BC values given are the instantaneous values at the corresponding speed. In order to get the average G7 BC for a trajectory, you average the BC's over the flight range of interest.

velocity	G7 BC
3300	0.255
3200	0.254
3100	0.253
3000	0.252
2900	0.250
2800	0.249
2700	0.247
2600	0.245
2500	0.243
2400	0.241
2300	0.239
2200	0.236
2100	0.234
2000	0.232
1900	0.231
1800	0.230
1700	0.230
1600	0.231
1500	0.233
1400	0.243
1300	0.242
1200	0.223

The average G7 BC from 3300 to 1500 fps (roughly the range of flight speeds over 1000 yards for the 300 RUM) is .241. The average for the lower velocity round (2800 to 1200 fps) is .237. So everyone who've been looking to velocity effects to explain the difference in BC is right that there is a difference, but according to the numbers above, the difference is only 1.7% for the average BC, not the 15% that's implied by the test. Could my numbers be off a little? Sure. But I don't think they're off that much. The implied .290 BC is coming from somewhere else.

Some more context. I've tested the 155, 168 and 178 Amax's. These bullets have essentially the same ogive and boat tail, which dictates that they should all have close to the same G7 form factor. The measured form factors for these 3 bullets are: 1.100 for the 155, 1.101 for the 168, and 1.118 for the 178. In other words, for bullets that have the same geometry as it relates to drag, they all have essentially the same measured form factor (drag) within 2%. Note the 178 has a little more drag which might be due to the longer bearing surface (more skin friction drag). The G7 BC's of these bullets are calculated by dividing the sectional density by the form factor. A G7 BC of .290 for the 178 Amax implies a G7 form factor of .924! This gets back to the notion that the .290 BC implies a level of drag that's simply unrealistic for a bullet shaped like this (and the 155, 168, etc).

I can think of many scenarios that would cause a bullet to fly with more drag than it should (stability problems, rough/worn barrels, bullet deforming in the barrel, etc). However, there is no way a bullet flies with less than it's minimal drag.

So we're back to the testing. I'm interested in Michaels answers to the questions above. My interest lies in understanding (for myself and helping others to understand) the science of ballistics. How and why some observations appear to contradict what's known about ballistics.

Those who deliberately enter false values into a ballistics program in order to get it to match what they shot in the field will get an answer that is more or less useful. But those who go to the trouble of verifying all the inputs are correct will get a prediction that is much closer to reality, is much more accurate over a wider range of conditions and distances, and will be more accurate on the other outputs (retained velocity, tof, wind drift, etc). If you're happy to knowingly enter false values, so be it. It's not my job to convince everyone that science works. But for those who are interested in getting to the bottom of things and doing it right, let's talk.

-Bryan

I hear you loud and clear about wrong data being less than usefull. I have said through out this thread that adjusting velocity does not work for me as I need to know impact velocities to know if a bullet will expand at the desired range to the game. When double chronies are used 200 yards apart and drop tests are scary close to the results where all of the other variables are accounted for (pressure, temp, humidity, scope height etc etc.....what else can a guy do?

It just seems like a consistent corosponding rise in BC to velocity over 3 different velocities.

If anything, I think you have prompted me to get my old 300RUM barrel to the smith and have the Edge barrel taken off and the RUM barrel replaced. It still holds a reasonable degree of accuracy. I will do another double chrony test and maybe a sound tests with a lap top for sh$!s and grins. I will send you the results and you can calculate a BC.

I will take the open mind approach on this. I hope you will as well.

Respectfully,

Michael

PS:

I know more than a few guys who have used and do you the 178 extensively. None of them can get your numbers in the JBM calculators to work. It is not just me.

 

[BryanLitz]

But the trigger was pulled and the results are the results.

The observations are what they are. If the observations contradict physics, do you question the observations, or the natural world as we know it exists?

Remember for thousands of years people believed the earth was flat and at the center of the universe because that's what was directly observed. It was actually punishable to believe any different (Galileo was nearly put to death by the church for pointing out that the earth was not the center of the universe).

My point is that sometimes observations can be misleading. In some cases, common sense and direct observation can get you by. In other cases, the truth is more elusive than we'd like it to be.

The only purpose in my statements of different rifles is how they relate to velocity differences, which is not a waste of time.

I've addressed the amount of velocity dependence that can be expected in the G7 BC of this bullet and the speed ranges involved. Consider that the velocity range of the .308 (~2800 to 1200 fps) and the 300 RUM (~3250 to 1500 fps) is about 80% common. It's not even a huge difference in average velocity. By the time the bullet from the 300 RUM has traveled 200 yards, it's already slowed to the approximate MV of the .308 Win. Point being the bullets aren't spending most of their flight at completely uncommon velocities, in fact, they overlap more than not.

Thank you for answering the question about testing in no/low wind. That's important to getting accurate results from a drop test.

Yes. .290 was a value that worked flawlessly over the course of the barrel's use including some long range kills albiet the tests were done to 1000 where it was only fired to 800 after that with machine precision.

Not quite clear to me how many tests were done that determined / repeated the .290 BC. Citing long range kills is compelling, but if you're talking about 600/700 yards, you can be off on your BC by quite a bit and still 'kill' an animal. A .290 BC vs a .240 BC only means ~5"-7" of error at 600/700 yards at 300 RUM velocities.

It's good that you account for the distance between the muzzle and chrono. I'd like to address your double chrono test. The difference between a .240 G7 BC and a .290 G7 BC over 200 yards is about 69 fps (17% of the total velocity lost in 200 yards). You mentioned earlier that the chronos are within 30 fps or less. I assume you determined this by firing thru them in tandem (one right behind the other) which is a good way to compare chronos. May I ask, when you then spread the chrono's apart by 200 yards, did they still share the same light conditions? Or was one now in the shade and the other in the sun? Just asking because I know chronos are exceptionally sensitive to light conditions. They might agree in the same light, but disagree in different light.

Same rings, same scope. Obviously the rails were different since one was a long action and one was a short action. Both (at the time) were picatinny 20MOA rails. Scope is calibrated to .2645" per click (.2526MOA). My personal software accounts for the correction. I use the same scope for tests because I have not calibrated any otehr scope. It's a pain and it is easier to switch the scope for tests and experiments.

I understand your reasoning here, but my real question is: did you ever calibrate that scope on the specific rifle that you did the drop testing with (the 300 RUM)? Or was it calibrated on another rifle and the same calibration factor applied when mounted to a different rifle? I don't know for sure that a scopes calibration factor would change when mounted to different rifles, but it would be a mistake to assume anything where measurement is involved. Especially when those measurements appear to imply non-physical results.

I hear you loud and clear about wrong data being less than usefull. I have said through out this thread that adjusting velocity does not work for me as I need to know impact velocities to know if a bullet will expand at the desired range to the game. When double chronies are used 200 yards apart and drop tests are scary close to the results where all of the other variables are accounted for (pressure, temp, humidity, scope height etc etc.....what else can a guy do?

If the observations imply a non-physical metric (like BC), a guy can either use the faulty data and hope it doesn't bite him, or continue searching for the reason why.

It just seems like a consistent corosponding rise in BC to velocity over 3 different velocities.

Yes, it seems like that, but what if there is another error in the system that's sensitive to velocity? Just about everything you measure in this exercise will result in more 'implied' error for faster MV's. Chrono's for example, have error that tends to be proportional to the bullet's speed. Furthermore, a slower MV will produce a trajectory with more drop. Since drop is what you're measuring, you can measure it more accurately if there's more of it, thereby allowing a higher resolution measurement. Errors in range produce more error in derived BC for a faster moving bullet than a slower moving bullet.
Combine the above facts with the fact that your lower velocity round matched my BC closer than the faster round and you could conclude either:
A) there is extreme (unrealistic, really) sensitivity of the actual BC to velocity, or
B) something else about your procedure/measurement is sensitive to velocity.

If anything, I think you have prompted me to get my old 300RUM barrel to the smith and have the Edge barrel taken off and the RUM barrel replaced. It still holds a reasonable degree of accuracy. I will do another double chrony test and maybe a sound tests with a lap top for sh$!s and grins. I will send you the results and you can calculate a BC.

What you do is up to you. It would be educational to chase this down for the sake of science, but it seems like it might jam you up to have your Edge unavailable. Furthermore, if you sent me raw data from one of your tests, I would probably derive the same result as you. I believe you know how to process the raw data into a BC properly. I believe the problem is with some aspect of the raw data. Something is being overlooked, or taken for granted. This is not intended as an insult to your abilities. I've effed up plenty of tests by overlooking, assuming, forgetting to measure or writing things down accurately or at all, etc. It's part of the learning process, which we are all constantly learning. If (when) one of my tests produces a result that I know to be impossible, I don't just shrug and say it must be magic, I search for the cause. Usually I find it. If I can't find any reason for it, I still don't think it was magic, I just conclude that I was unable to find the source of the error, and re-test. Invariably the 'magic' result is not repeated in a properly done test.

PS:

I know more than a few guys who have used and do you the 178 extensively. None of them can get your numbers in the JBM calculators to work. It is not just me.

Be careful, as you DID repeat my result within 3% with one of your rifles. So in that case, two independent tests (one of them yours) produced the same result. Your .290 result is a case of not being able to reproduce the same performance in two rifles (I know that your point is that the performance actually is different, and that's what I'm arguing).

Also, there are many shooters who report that my BC's are accurate, and have helped them get on target at long range (including those who shoot the 178 Amax). Of course there are exceptions, but are we to conclude that those exceptions to the rule are the only one's doing it 'right', and the majority of shooters who hit targets with my BC's are wrong?

I agree to take an open mind approach to this as well. Out of curiosity, where do you live? I would really enjoy the opportunity to meet up and do some testing together. That would be the best way (maybe the only way) to make the necessary discoveries.

-Bryan

 

[Michael Eichele]

See bold below:

The observations are what they are. If the observations contradict physics, do you question the observations, or the natural world as we know it exists?

If I am wrong, then my 'observation' condradicts physics. If I am right then there are variables that are not known that need to be accounted for. Physics are great if all the variables are accounted for. Either I am dead wrong or there is another variable that is not known.

Remember for thousands of years people believed the earth was flat and at the center of the universe because that's what was directly observed. It was actually punishable to believe any different (Galileo was nearly put to death by the church for pointing out that the earth was not the center of the universe).

My point is that sometimes observations can be misleading. In some cases, common sense and direct observation can get you by. In other cases, the truth is more elusive than we'd like it to be.

I could say the same and apply that to you. No disrespect meant. Maybe I could be on to something and flammed here like Galileo was nearly executed. Dont think for a second though that I am comparing myself to Galileo. Just showing you how your illustration could be looked at from my percpective.


I've addressed the amount of velocity dependence that can be expected in the G7 BC of this bullet and the speed ranges involved. Consider that the velocity range of the .308 (~2800 to 1200 fps) and the 300 RUM (~3250 to 1500 fps) is about 80% common. It's not even a huge difference in average velocity. By the time the bullet from the 300 RUM has traveled 200 yards, it's already slowed to the approximate MV of the .308 Win. Point being the bullets aren't spending most of their flight at completely uncommon velocities, in fact, they overlap more than not.

Thank you for answering the question about testing in no/low wind. That's important to getting accurate results from a drop test.



Not quite clear to me how many tests were done that determined / repeated the .290 BC. Citing long range kills is compelling, but if you're talking about 600/700 yards, you can be off on your BC by quite a bit and still 'kill' an animal. A .290 BC vs a .240 BC only means ~5"-7" of error at 600/700 yards at 300 RUM velocities.

It's good that you account for the distance between the muzzle and chrono. I'd like to address your double chrono test. The difference between a .240 G7 BC and a .290 G7 BC over 200 yards is about 69 fps (17% of the total velocity lost in 200 yards). You mentioned earlier that the chronos are within 30 fps or less. I assume you determined this by firing thru them in tandem (one right behind the other) which is a good way to compare chronos. May I ask, when you then spread the chrono's apart by 200 yards, did they still share the same light conditions? Or was one now in the shade and the other in the sun? Just asking because I know chronos are exceptionally sensitive to light conditions. They might agree in the same light, but disagree in different light.

If you have read all of my posts, you would find where I have addressed lighting issues. That said, I will cover it again. I tend to shoot in various lighting conditions in general. However, when I need numbers that really matter for calculation or test purposes, I will do it on overcast days. We get alot of thick heavy overcast days here so I tend to have more opprotunites there. The benefit to that (unless I am way off base here) is that it lessens the inconsistencies due to light factors. Tandem? Yes. Both at 15' and then again at 200 yards. Are they always 30 (average is actually 32)? No. It is a close average. It also fluctuates a bit depending on the velocity spectrum I am on. I do the best I can whith what I have. Believe me I am well aware that one in the shade versus one in the sun is deceiving. It is WAY worse when one photo eye is in the sun and one in the shade. Range canopies and changing sun angles are notorious for this during a range session. Some guys are not that observant and scratch their heads. I am not here to pick on them so I will move on.



I understand your reasoning here, but my real question is: did you ever calibrate that scope on the specific rifle that you did the drop testing with (the 300 RUM)? Or was it calibrated on another rifle and the same calibration factor applied when mounted to a different rifle? I don't know for sure that a scopes calibration factor would change when mounted to different rifles, but it would be a mistake to assume anything where measurement is involved. Especially when those measurements appear to imply non-physical results.

Again, if you read through all of my posts you will find that I did not use any rifle to 'calibrate' my scope. It was attached to a bench rest.



If the observations imply a non-physical metric (like BC), a guy can either use the faulty data and hope it doesn't bite him, or continue searching for the reason why.

Definatley looking for any reasons I may be doing this wrong. If I am I would love to change my methods for accuracy purposes. I want my system to be as near 100% as possible. I accept the fact that it will likely never be 100%. As close as possible has to be acceptable.



Yes, it seems like that, but what if there is another error in the system that's sensitive to velocity? Just about everything you measure in this exercise will result in more 'implied' error for faster MV's. Chrono's for example, have error that tends to be proportional to the bullet's speed. Furthermore, a slower MV will produce a trajectory with more drop. Since drop is what you're measuring, you can measure it more accurately if there's more of it, thereby allowing a higher resolution measurement. Errors in range produce more error in derived BC for a faster moving bullet than a slower moving bullet.
Combine the above facts with the fact that your lower velocity round matched my BC closer than the faster round and you could conclude either:
A) there is extreme (unrealistic, really) sensitivity of the actual BC to velocity, or
B) something else about your procedure/measurement is sensitive to velocity.



What you do is up to you. It would be educational to chase this down for the sake of science, but it seems like it might jam you up to have your Edge unavailable. Furthermore, if you sent me raw data from one of your tests, I would probably derive the same result as you. I believe you know how to process the raw data into a BC properly. I believe the problem is with some aspect of the raw data. Something is being overlooked, or taken for granted. This is not intended as an insult to your abilities. I've effed up plenty of tests by overlooking, assuming, forgetting to measure or writing things down accurately or at all, etc. It's part of the learning process, which we are all constantly learning. If (when) one of my tests produces a result that I know to be impossible, I don't just shrug and say it must be magic, I search for the cause. Usually I find it. If I can't find any reason for it, I still don't think it was magic, I just conclude that I was unable to find the source of the error, and re-test. Invariably the 'magic' result is not repeated in a properly done test.

I dont believe in magic either. I do believe in the sum of the parts working together.

I search too when 'standards' dont match my findings. I hate to admit this but I had initially thought the 178 was even higher in the RUM. Much higher. So much so that I thought my scope (new at the time) was defective. I set it up at the range only to find that the reticle was spot on and the clicks were very close. So close that it didnt even come close to accounting for the high BC. I accepted it as they way it was. Not long after that, you said something (in another thread) that promted me to check again. Having not removed my rings from the scope, nor the base from the rifle, the scope had been reinstalled on the rifle. It turned out that my zero was not perfect and I screwed up the altitude/pressure fields in my software. Hence the reason I now only use raw pressue and a 0' altitude. It is also why I 'fool proofed' my software so that it could not happen again. In any event, between the 2 errors I came up with a very large BC. This is why I spend so much time and effort testing this bullet (178). In the end, I ended settling on the figures we have been disscusing here.


Be careful, as you DID repeat my result within 3% with one of your rifles. So in that case, two independent tests (one of them yours) produced the same result. Your .290 result is a case of not being able to reproduce the same performance in two rifles (I know that your point is that the performance actually is different, and that's what I'm arguing).

Also, there are many shooters who report that my BC's are accurate, and have helped them get on target at long range (including those who shoot the 178 Amax). Of course there are exceptions, but are we to conclude that those exceptions to the rule are the only one's doing it 'right', and the majority of shooters who hit targets with my BC's are wrong?

Yes I did repeat your result within 3% with one of my rifles. In that exact same rifle and scope with a new barrel and new load the velocity was up 200 FPS with a similar corosponding increase in BC. It shows a pattern and consistency.

I am one of the many shooters you speak of. I too have had some of your JBM values work scarey close for some bullets. Just not most of them. I have no intentions on making the exceptions the rule. I do however have intentions on showing that there are no 1 size fits all for all velocities and/or bore qualities, stability factors etc....and how they all tie together. Yes I realize some of these equate to small differences. Total all of the variables up from one end of the spectrum to the other and you will find some extreme exceptions. That does not make your values wrong. It is hard to be wrong or right when there are so many variables with everbody else's systems. IMHO BC's published by you are anybody are only a starting point. Every now and then, they fit perfectly (or within a fraction of a percent).

I agree to take an open mind approach to this as well. Out of curiosity, where do you live? I would really enjoy the opportunity to meet up and do some testing together. That would be the best way (maybe the only way) to make the necessary discoveries.

-Bryan

I live in Anchorage Alaska. I would love nothing more than to bounce what I have been doing off of you. I am many more times interested in getting any 'bugs' ironed out of my shooting systems than I am about just being right. Sure I dont like being wrong. Who does? I can live with and accept being wrong even if I dont like it. I would rather be correct in my system. All I ever ask is that I be shown what I am doing wrong.

Ever do any hunting or fishing? Maybe you could kill to birds with one stone. Deem it as an experiemnt and maybe you could write it off of your taxes? I would have a hard time making it down to the lower 48 any time soon.

All things concidered, I am not worried about not having my Edge available for a while. It is a switch barrel gun. The barrel can be re-installed in a pinch. I just need my smith to take them on and off. I dont have a barrel vice or action wrench. I wont worry about the scope either. It will be removed and the scope I do my testing with will be installed and sighted. Besides, I only use it for hammering large animals at long range. I dont plan on that until August. I mainly need to take it to my smith not only to take off the Edge barrel but also to have him re-head space the 300. Even though it is a switch barrel rifle, the bolt lugs were relapped a few thousandths at one point after the Edge was fitted and head spaced and then re-head spaced.

After I re-do everything, if I come up with totally different results than before and they match more closely your figures, I may be scratching my head about it and will loose alot of sleep trying to figure out where I went wrong in the past and why the system worked so well. If the results are the same as before, then we will have some serious work to do. Either way, I will post the variables and the results. It will be 1-2 weeks before I can get it done. Much of which will depend on the cloud cover. If there is still a big question mark I would even be willing to let you borrow that barrel if you have something to attach it to. Even give you the load data.

Michael

 

[Jon A]

I think what your not getting is that some of us do all of the things you suggest.

Respectfully, that doesn't seem to be the case. I explained to you why measuring a scope over only 10 MOA was not adequate. When you crank 50 MOA or so into it you really have no idea exactly how much it is adjusting. The last thing in the world you should do is assume it's a calibrated instrument with which to accurately measure drop at a mile and from that backward calculate an accurate BC of the bullet.

My main concern in setting up a drop chart is producing numbers from the data that exactly fit the flight of the bullet. That bullet comes with a specific shape that gives it a specific BC. To be useful to the gun operator, the bullet is exposed to a huge explosion and high pressure and is forced down a tight twisted tube, putting it under extreme stress. If that bullet shape changes in the slightest amount from those forces, so does the BC .

Apologies for not realizing who you were or who you work for the first time around. To re-cap, these instances in which an identical rifle, ammo and velocity resulted in much different trajectories in the same conditions were observed with a different Huskemaw scope on the top of each one? Yeah. The next time this happens, could you please remove the Huskemaw scopes and bolt a high end tactical scope on top of the rifles and re-test (you can even use the same scope). You just might be amazed at how even a scope change will affect the BC of the bullet.

IMHO, it is a waste of time. The difference works out to about 2.5" at 1K when dialing 300" worth of drop. It would only take a small fraction of BC to make up for it. For all I know, the differences stem from inaccuracies on my part which may explain why there was a difference of 0.05" between the upper adjustment and the lower. That said, since it was such a pain in the ***, I have never done this test to my Nightforce.

It is never a waste of time. Even if you confirm a scope is accurate, you only know that and can have confidence in it after the fact. There are much easier ways to do it which I described in the optics section. It's so easy there's no reason not to do it. The amount of ammo and head-scratching it can save makes it well worth the effort.

.248 to .290

Is that a steep change? Yes. It is what it is. The velocities were checked again and again. the zeros were checked again and again. The drops were checked again and again. All the variables were accounted for and these were the results.

Have you ever actually measured such a difference shooting over your dual chronograph setup on the same day with the same setup? Do THAT and you'll really have something worth arguing over.

 

[groper]

Yes I did repeat your result within 3% with one of my rifles. In that exact same rifle and scope with a new barrel and new load the velocity was up 200 FPS with a similar corosponding increase in BC. It shows a pattern and consistency.

To me, it shows a consistent pattern of error in your methods or equipment, not a consistent error in the physics. Bryan highlighted the fact earlier, that the lower velocity trajectorys have more drop, and the increase in drop gives you a corresponding increase in measurement resolution - This is what forced you arrive at a more accurate result, not the velocity itself.

The flatter trajectories have less resolution, and are therefore more prone to letting small errors creep in without being noticed.

Eddy, ive tried to drop test and derive BCs before, i always ended up with a BC different to what Bryan Litz published for the all the long range bullets ive used in 6.5mm, .30cal and .338cal. After i read about how Bryan derives his BC`s, i switched to using them, and repeated all my testing. What i found is, the velocity i was getting from my chrony caused most of the difference, and when i changed the assumed velocity, my trajectories now match better than they ever did before, scary accurate...

So why we keep trying to second guess a more accurate method like Bryans, with our own "home grown" methods, is beyond me. Drop testing is not a good way to derive a BC... Why try to control errors, when you can remove them entirely? Please try the accoustic method -it removes all the scope errors, aiming errors and drop errors. The chrony error is still there, and so are the environmentals and distance errors, but these are much easier to deal with provided careful attention is paid - as i outlined earlier in the thread.

 

[Michael Eichele]

To me, it shows a consistent pattern of error in your methods or equipment, not a consistent error in the physics. Bryan highlighted the fact earlier, that the lower velocity trajectorys have more drop, and the increase in drop gives you a corresponding increase in measurement resolution - This is what forced you arrive at a more accurate result, not the velocity itself.

I would accept that if all of my tests resulted this way. However, the overall consistency has show that even you would agree with many of my results. To me, it does not show a consistent pattern of error. If anything, it shows a consistency of reuslts that most could agree on with a few exceptions.

If I have been so inconsistent, ask Bryan what G1 value he came up with for the 208 AMAX at 2900. Then compare to mine (.671). I would bet you will see a VERY small difference. Or the 190 SMK at 2660, or the 300 Hybrid at 2750 so on and so forth. I bet I have been consistent enough to scare the he!! out of you.


So why we keep trying to second guess a more accurate method like Bryans, with our own "home grown" methods, is beyond me. Drop testing is not a good way to derive a BC... Why try to control errors, when you can remove them entirely?

You're right. It is beyond you so why hound me about it. At least I get out there and shoot, test, experiment, prove, dissprove, help and be helped all while honing my skills and getting good data to validate my trajectory programs even if it is at the risk of scrutiny, flame or what have you..... Nothing ventured, nothing gained. I for one would not have it any other way. If I am wrong here then when that becomes evident and I change my methods I assure you that I will have learned a heck of a lot more along the way from pure range time and experimenting. I will have more peices of the puzzle than the average guy. Is that neccesary to long range hits? Not much but I still enjoy the experience, range time and practice.

I am sorry that it is beyond you.

Drop tests, if done right are a great way to assist in determining a good BC. Is it the only way that should be used? If that is all you have, maybe. Is it the best? No. Thank goodness there are other ways to do it. In my opinion for the average guy who does not have access to the good stuff, double chronies and drop tests are all we have.


Please try the accoustic method -it removes all the scope errors, aiming errors and drop errors. The chrony error is still there, and so are the environmentals and distance errors, but these are much easier to deal with provided careful attention is paid - as i outlined earlier in the thread.

I am working on it. Let you know what I find.

Until then, get off my case. Unless you want to take a stab at getting out there and trying it for yourself. What is the worst that can happen? Learning something?

At this point, I will test the 178 in my 300 one more time. If wrong, you and Bryan will be the first to know. Till then, you do it your way and I will do it my way.

 

[Michael Eichele]

Have you ever actually measured such a difference shooting over your dual chronograph setup on the same day with the same setup? Do THAT and you'll really have something worth arguing over.

I will do it again. Soon. In fact, I will do it with the 308 and the 300 together.

Till then I will shut up about it. After that, if not the same as before, I will post the numbers. If I am wrong I will accept that publicly. If nothing has changed, we can go from there.

Fair enough?

M

 

[BryanLitz]

Michael,

Since you're going to shoot that barrel some more, I have a couple more thoughts to share about your next tests.

1) On re-reading some of your posts, I get the impression that you have not directly measured your scopes actual POI shift (by shooting it) while mounted to the rifle in question. I understand that bench testing the scope 'ought to' produce the same result, but if we're really serious about this test, it's something that has to be verified by actually firing a group, adjusting the turrets, and firing another group. I've provided some guesses as to why/how the mounting might affect the calibration factor of a scope, neither of which I'm sure are true/possible. Point is, until that loop is closed, we're making an assumption about a critical part of this test (scope tracks as you think it does).
One potential way to circumvent this error is to put your 1000 yard elevation on the scope and shoot a group at 100 yards on a tall target. Measure the height of the impact above the aimpoint. Without touching the turrets (so as not to introduce that variable) shoot the rifle at 1000 yards and note the impact there.

2) Another potential error I've seen crop up in tests has to to with the way the rifle is supported when establishing the 100 yard zero, as opposed to how the rifle is supported when shooting the longer ranges. It's well known that gun handling is important to accurate shooting, which implies that the POI of the rifle can change based on how it's held when firing. For example, if the 100 yard zero is established from a bench with a cast iron pedestal rest and sand bags, then you lay down and shoot the rifle with a bi-pod for the longer ranges, there may be a POI shift of a couple MOA that you can't possibly know exists.

To be clear, none of this is intended to insult your intelligence, just brainstorming on the possibilities.

-Bryan

 

[Michael Eichele]

See bold below:

Michael,

Since you're going to shoot that barrel some more, I have a couple more thoughts to share about your next tests.

1) On re-reading some of your posts, I get the impression that you have not directly measured your scopes actual POI shift (by shooting it) while mounted to the rifle in question. I understand that bench testing the scope 'ought to' produce the same result, but if we're really serious about this test, it's something that has to be verified by actually firing a group, adjusting the turrets, and firing another group. I've provided some guesses as to why/how the mounting might affect the calibration factor of a scope, neither of which I'm sure are true/possible. Point is, until that loop is closed, we're making an assumption about a critical part of this test (scope tracks as you think it does).
One potential way to circumvent this error is to put your 1000 yard elevation on the scope and shoot a group at 100 yards on a tall target. Measure the height of the impact above the aimpoint. Without touching the turrets (so as not to introduce that variable) shoot the rifle at 1000 yards and note the impact there.

I will concider re-visiting this

2) Another potential error I've seen crop up in tests has to to with the way the rifle is supported when establishing the 100 yard zero, as opposed to how the rifle is supported when shooting the longer ranges. It's well known that gun handling is important to accurate shooting, which implies that the POI of the rifle can change based on how it's held when firing. For example, if the 100 yard zero is established from a bench with a cast iron pedestal rest and sand bags, then you lay down and shoot the rifle with a bi-pod for the longer ranges, there may be a POI shift of a couple MOA that you can't possibly know exists.

Agreed. It is for this reason that I typically do double chronies off the bench at my local range where drop tests are done in the field using the same gear I will have hunting with. Bi-pod and rear support.

You are probably cringing right now. So be it. I seem to do well with bi-pods on the ground despite the fact that many guys swear them off.

My drop tests are done laying on the ground with the pod and rear bag. Yes I check my zero each time before I start the long game. If there are any questions or doubts afterwards, the zero will be checked again. Ever since I screwed up that one test with a bad zero I ALWAYS verify and adjust need be before I begin.

Also, I do not use a 100 yard zero for any testing purposes. I start with a 300 yard zero and work out. That is not to say that I never use or carry a 100 yard zero. I just test from 300 to 1000 and the 200 and 100 are what they are. You are probably cringing again. So be it.

I realize I have alot of 'answers' when questions come up. I really am very detailed minded.

To be clear, none of this is intended to insult your intelligence, just brainstorming on the possibilities.

-Bryan

Likewise.

The thought never crossed my mind. If I am doing something wrong, I want to fix it. If you or anybody else can help me figure out what is wrong, or why it is working the way it is, I welcome it.

That said, I am going to let this rest. After I get my rifle back from my smith and get her to the range, I will post another thread. Who knows, maybe I'll learn something new. If I can duplicate the results, we will go from there. Either way, you will all know what happens.

Michael

 

[Jon A]

I will do it again. Soon. In fact, I will do it with the 308 and the 300 together.

That would be great. Remember to calibrate the chronographs to each other before hand and post all your raw data.

2) Another potential error I've seen crop up in tests has to to with the way the rifle is supported when establishing the 100 yard zero, as opposed to how the rifle is supported when shooting the longer ranges. It's well known that gun handling is important to accurate shooting, which implies that the POI of the rifle can change based on how it's held when firing. ...

Yes, one of the many reasons I simply don't believe drop tests done at short-mid ranges have any business being used to estimate BC's.

Using that as an example, let's say since you're shooting at a different target, are using different or have adjusted the rest, you're pointing in a different direction, or whatever the reason, your failure to hold the rifle exactly the same way results in a 1/2 MOA POI error. That's not very much and is very easy to do--especially with a hard kicking rife.

Using the same conditions as listed before (5000 ft, 3200 fps, 300 yd zero), the 1/2 MOA hold error results in 3.9" of elevation error at 750 yds. The difference in drop at 750 yds from a .700 G1 BC and a .850 G1 BC is only 4.1". So this simple shooting error will completely erase the rather large difference between a .850 BC and a .700 BC bullet. Or it could double the error making you think the difference between the bullets is twice what it really is.

Or conversely if you're testing the same bullet it's enough to make you think the BC changed over 20%!!! for some mysterious reason!

Now let's double the range to 1500 yds. The 1/2 MOA hold error results in 7.85" elevation error. The difference between a .700 and .850 BC bullet is 54.4". You can't miss that!

Now the difference in BC between the two bullets is seven times as big as the hold error. The hold error is still there, but there's no way you're going to completely miss such a large BC difference because the hold error can no longer erase or double it. Most of the difference in drop will be due to the BC difference, not the hold error.

Conversely, if testing the same bullet there's no way the hold error is going to make you think the BC of the bullet magically changed 20%+ for some unknown reason.

That's just one of the many variables at work. Velocity errors are relatively dominant for close range trajectories and become less so at longer ranges relative to BC. Scope click error becomes more important at long range but that's easy to measure and correct for.

This is why measuring drop at close ranges with flat shooting rifles is such a fundamentally incorrect way to estimate BC's. All the other little errors affect trajectory so much the small amount a due to a BC difference is lost in the noise. If you want to look at drop, it needs to be done at really long ranges.

Even 1000 yds is not enough. Well, maybe it's OK for a 6.5 Grendel, but certainly not an ultramag shootings high BC bullets fast.

 

[WildcatB]

Another bone to chew on.... it seems that the longer the barrel the closer to .9 bc g1 is needed to get the drops to match up.

I just emailed a person who shoots them out of a pistol and he needs .75 g1 to get his drops to match.

On JBM's website when you check "Elevation Correction for Zero Range " - the software determines the vertical angle (angle of departure) based on your zero distance. This introduces some error since it doesn't ask for your barrel length. I asked JBM about this. He acknowledged that there was some error here, roughly the barrel lenght multiplied by the vertical angle result. But he thought it was too insignificant to make a difference.

I shot the bullets yesterday at 1000 using .74 with the correct scope height (measured at the chamber) and they still landed 6 to 7 inches high - a .84-.85 put me right on. It's not .9 but not .74 either.

Bryan, you know the equations for this....I couldn't find them anywhere. What exaclty is the error and how much difference in elevation would the corrected departure angle give you at 1000 & 2000 yards using 18, 24, 28, 30, 32 inch barrels?

 

[ericpetritz]

2) Another potential error I've seen crop up in tests has to to with the way the rifle is supported when establishing the 100 yard zero, as opposed to how the rifle is supported when shooting the longer ranges. It's well known that gun handling is important to accurate shooting, which implies that the POI of the rifle can change based on how it's held when firing. For example, if the 100 yard zero is established from a bench with a cast iron pedestal rest and sand bags, then you lay down and shoot the rifle with a bi-pod for the longer ranges, there may be a POI shift of a couple MOA that you can't possibly know exists.

-Bryan

This is a very good point that should not be taken lightly. I saw this a few months ago. To make a long story short, the shooter was shooting amazing groups at 200 yards, then moved to the 500 yard target and the bullets were impacting much higher than predicted. After confirming inputs, finding nothing wrong, and scratching our heads, another shooter fired the same two targets with the same rifle and the groups printed exactly where they were predicted to. Eventually we determined the problem was that the first shooter was squeezing the rear bag very hard on the short range target and was relaxing the rear bag to gain the proper sight picture on the far target. When he fired the far target, the rifle butt sank into the rear bag and launched the bullets high. The problem was made worse because it was a heavy recoiling rifle. After he realized he was doing this, he concentrated on maintaining a consistent hold on the rear bag and then he started putting the bullets "right on the money". This problem would have appeared even worse as the range increased.

Just something else to think about.....

 

[BryanLitz]

Paul,
see the table below for an answer to your question about the effect of barrel length on sight height. This assumes 2800 fps for your 200 grain bullet, and a rifle that has a sight height of 2.00" measured at the base of the barrel (between the turrets and the objective).

drop at 1000 and 2000 yards is -280.29" and -1996.04" for a sight height of 2.00".

bbl	effective	drop@	drop@
length	sight height	1000	2000
18	1.84"		281.73	1999.08
24	1.79"		282.18	2000.03
28	1.76"		282.45	2000.60
30	1.74"		282.63	2000.98
32	1.72"		282.81	2001.36

So, the worse case scenario, with a 32" barrel, the effective sight height is reduced by .28" (from 2.00" to 1.72"). This causes a difference in predicted drop at 1000 yards of 2.52", and 5.32" at 2000 yards.

The 2.52" error at 1000 yards translates to a BC of 0.013 (1.8%) less than the actual value. Note that by accounting for the true sight height at the muzzle, there is actually more drop. The error is less for shorter barrels, but the error would always imply a lower BC than actual, not a higher BC.

I'll say this another way because I know it's confusing. When you run a 1000 yard trajectory with an 'uncorrected' sight height of 2.0" that you measure at the turrets (for example) you will expect -280.29" of drop at 1000 yards. With a 30" barrel, the actual, effective sight height will be 1.74". This sight height will actually produce a trajectory that has 282.63" of drop. This is more than the 280.29" you were expecting, so it would make you think the BC is lower (but only by a little bit).

This was a useful exercise to go thru, but I think it demonstrates what Brad told you: effects of sight angle on sight height are small.

Thinking about that calls into question many things about how we enter variables into ballistics programs. The program runs as if the trajectory, and angular adjustments to the trajectory originate at the same point. In reality, the bullet emerges from the muzzle, which is forward of the point where the angular adjustment originates (scope turrets). If someone ever chased all that down, I suspect two results: A) they would have a headache, and B) they would conclude that there is negligible error introduced by the length of the barrel.

When you measure the range to the target, is it the range from the scope turrets, or the range from the muzzle? I don't care. It won't affect anything that's measurable about a long range trajectory.

Let me point out some things that I DO think can more easily cause a shooter to perceive more error in a 'drop derived' BC for your bullet.

1) range error. The bullet should have 282.81" of drop over 1000.0 yards. If there is -10 yards of uncertainty in the range measurement (-1% error), you will observe only 275.47" of drop. That will imply a BC of .74; a 6% error.
2) confusing station pressure with corrected pressure. If someone's shooting at 5000 feet altitude, and they 'mix up' station and corrected pressure either by misinterpreting their source, or entering it incorrectly into a program, it might tell them they should expect the 282.81" of drop, but (since they're at 5000 feet) they will observe only 261.25" of drop. This implies a BC of .84; a 20% error.
3) chronograph error (many kinds). Each 10 fps of chrono error corresponds to about 2% of BC error (in this particular case). Note that the bullet looses 7 fps from the muzzle to chrono (was this accounted for?). Also consider that if the chrono is angled slightly in any direction, it will artificially reduce the measured MV. Note that for a 5 degree angle, the chrono will read about 11 fps slower. Both of the above errors artificially drive the perceived MV down, which makes the bullets appear to be flying with a higher BC based on their observed drop.

The list goes on, but it's getting late here. Point is, there is a long list of potential error sources in BC testing, especially when the test involves measuring drop. You yourself have conducted 3 tests which implied BC's of .74, .85, and .9; a span of 22%. That is clear, first hand evidence that the drop test is not repeatable (at least it's very difficult to execute it repeatably).

For those who are interested, here is some terrific reading on some more potential errors in ballistics programs:
Ballistic Program Inaccuracies

-Bryan