Let's argue about BC's

[ericpetritz]

While the wildcat are secant ogive they have a rebated boat tail and are termed as uld.

Yes, I understand the terminology. The point is that the G7 curve and G7 BC's are a better match (less velocity dependence) for VLD's or ULD's than the G1.

Michael,
Try this:
Start by assuming the Litz measured G7 BC of your bullet is accurate. Confirm the actual travel per click of your scope--the predicted drop values will have to be scaled by the error. Measure the sight height. Accurately measure the station pressure (not corrected), temperature, and relative humidity. Measure your velocity. Zero at short range--make sure this is accurate and not off by a click or two. Use the free ballistic software on Berger's website with your inputs. If after confirming all of this your drops do not match what is predicted, assume the muzzle velocity is incorrect and adjust it. There will typically be more error associated with a chronograph than with the measured BC. Try this method and let us know.

 

[eddybo]

I think what your not getting is that some of us do all of the things you suggest. Apparently do them well for some bullets, as some bc s work well. Yet with other bullets in the same rifles have huge disparities . I would venture a guess that in the last 4 years I have gotten a couple hundred rifles ready for long range work. All probably work as well as anyone else's rifles here. I doubt there are many hobbyists who have shot more long range than I have in that period. Still in many instances bc s are bonkers. I think maybe we are seeing instances where theory does not match reality. The stances of some make me wonder how much actual long range shooting they actually do.

 

[Michael Eichele]

Michael,
Try this:
Confirm the actual travel per click of your scope--the predicted drop values will have to be scaled by the error.

Done

Measure the sight height.

Done

Accurately measure the station pressure (not corrected), temperature, and relative humidity.

Done

Measure your velocity. Zero at short range--make sure this is accurate and not off by a click or two.

Done. Over and over.

Use the free ballistic software on Berger's website with your inputs. If after confirming all of this your drops do not match what is predicted, assume the muzzle velocity is incorrect and adjust it. There will typically be more error associated with a chronograph than with the measured BC. Try this method and let us know.

This has all been done. Over and over. I have used bergers program, JBM, RSI Shooting Lab, Barnes, and my own personally developed software among many others. Several of which are highly esteemed. When used properly, I get nearly identical results between programs. Some minor differences yes but VERY minor. Years ago I used station pressure corrected for sea level and inputed altitude. Now I use station pressure and forget the altitude. Either way, the results work out the same. I currently use station pressure only to simplify the system. I am not new at this game. If there is a viable explaination here, it isnt the obvious such as scope height, calibration, using the wrong pressure, temp or humidity etc......Those bases are covered.

Assuming that it is my chronies, why then does one bullet match Litz's within a fraction of a percent, another 3% and another 8% and another by 5%? Would you not think that if my chrony was the sole culprit and innaccurate by say 2% that this would be reflected through out all the testing and experiments that have been done? I could even live with a window of 2-3% but when it shows such a large gap, there is more to it than velocity errors. That does not explain why so many in this thread say the G7 is not affected by velocity yet when I use the same testing equipment and same methods using different velocities, they are different. Even with the SAME 'calibrated' scope.

I will offer this example as a basis for my skepticizm about innaccurate velocities being the sole factor here.

Using the 178 AMAX and 3242 FPS and a .290 BC matched my drops perfectly in any enviornment. To assume that the Litz BC is correct, I would have to adjust my velcoity to 3456 FPS. I doubt very seriously that this rifle was capable of shooting that bullet that fast with the powder that was being used. Between my personal chrony and a friends ohler, neither were close to the 3450+ mark. I am not saying there were some velocity innaccuracies at play here but I have a hard time believing that this is the sole reason. The gap is too large. Are chronies innaccurate? Yes, I believe they are. Are they close? I believe they are.

I am still looking for viable answers.

 

[Michael Eichele]

I think what your not getting is that some of us do all of the things you suggest. Apparently do them well for some bullets, as some bc s work well. Yet with other bullets in the same rifles have huge disparities . I would venture a guess that in the last 4 years I have gotten a couple hundred rifles ready for long range work. All probably work as well as anyone else's rifles here. I doubt there are many hobbyists who have shot more long range than I have in that period. Still in many instances bc s are bonkers. I think maybe we are seeing instances where theory does not match reality. The stances of some make me wonder how much actual long range shooting they actually do.

+100

I think this is one of the best posts yet in this thread.

M

 

[Topshot]

With regards to the Litz, G7 data.

If you look at the graph that comes with it in his book you will see that some of the data comes from only a small sample of shots. His G7 B.C. values are an average from this sample of readings. So may vary a bit from that obtained using a larger sample.

I believe that a B.C. value can change with atmospheric conditions, just like the speed of sound changes. The two are linked, especially down near the transonic barrier.

The amount of change will depend on the form factor of the bullet. Especially with regard to meplat diameter and boat tail angle and their effect on the pressure wave.

 

[groper]

Michael,

Measure you scope bore ht, verify your zero, verify all environmental conditions, adjust for the scope error your scope has per click value, use the litz G7 BC and lastly, adjust your trajectory fit by adjusting the velocity in the calculator to get onto center mass at AT 1000yds - dont listen to your chrony. I say 1000yds, because this is the distance Bryan Litz averages his derived BC`s across and is a large enough distance to achieve a decent resolution. I say dont listen to your chrony, as its apparent to me after reading your previous posts, this is definately a source of a good portion of the error. My chrony also gave me headaches...

So lets say after youve checked everything, you needed say exactly 70 clicks to get your groups center of the x at a target verified exactly 1000yds away in a no wind situation - wind throws your impacts vertical due to the magnus effect. Very light wind ok, wait for another day if the wind is strong. Keep adjusting the velocity in the calculator until it tells you 70clicks using Litz`s BC and the environmentals of the day. Your trajectory should now fit all the time, anywhere you shoot.

 

[MontanaRifleman]

I agree accurately measured BCs are great......but find that they are most accurately measured by the barrel you are going to be shooting that in.

I have on numerous occassions found one bullets BC to be very accurate, then when switching to another manufactuerers bullets in the same rifle found the BC to be very inconsistant with the trajectory of the rifle. A situation such as that, especially in a situation where ammo with both bullets are being shot through the same chronograph within a span of only a few hours is disheartening to one's beleif that BCs are infallable.
If you want to go past 1K or heck even to 800 BCs are not plug and play, the gun has to be shot.

I have very limeted LR BC testing which has been G1 only, and in the testing I did, there was no G1 BC that would fit.... so I fudged the velocity and BC to get the best curve I could to match my drops.

When you say you switched to another manufactured BC, are you talking G1 or G7 and are you using the manufacturers BC or BL's.

I simply do not trust comercialy availble chrony's based on my own experience with them. I have seen them change read outs, same load, same rifle, from 9 AM to 11 AM to 1 PM, etc., big time... and at twilight.. forget it.

No BC's are infallable, but Bryan's research ad testing is about as close as you are going to get and that's just the facts of life.

When manufacturers publish BC's, their profit margins are riding on those BC's. Bergers BC's actually dropped after Bryan's testing and he has been in these forums, very tranparently explaining his work and answering tough questions, not to mention the book he published which sheds a whole lot of light on the subject. Do you see any other manufacturer's balisticians in here doing the same thing?

All that being said, anytime we are using this kind of equipment we should understand that it is not infallable. Now and then any equipment will have hickups and so will the operators.

Yup, I agree, the gun always has to be shot, but the G7 BC's should be very close at most velocities and ranges... if they aren't... it should warrant some investigation.

 

[MontanaRifleman]

I think what your not getting is that some of us do all of the things you suggest. Apparently do them well for some bullets, as some bc s work well. Yet with other bullets in the same rifles have huge disparities . I would venture a guess that in the last 4 years I have gotten a couple hundred rifles ready for long range work. All probably work as well as anyone else's rifles here. I doubt there are many hobbyists who have shot more long range than I have in that period. Still in many instances bc s are bonkers. I think maybe we are seeing instances where theory does not match reality. The stances of some make me wonder how much actual long range shooting they actually do.

I think what you're not getting is... well... uhhh... I forget what you're not getting

 

[bigngreen]

Very interesting thread with a lot of good info to gain from!! I got the Audacity software loaded and tested on my laptop and the next time I hit the range I'll give it a try to see if I can get something useful out of it.

I've struggled with some bullets and not so much with others to get trajectories to match, I think most of my trouble revolve around using a chronograph and playing to much with BC's.
Learning to go through all the preliminary checking of the optic and "proofing" it made the process a whole lot better, also actually measuring the distance shot instead of just accepting that the range is set up correctly made a big difference.

The last couple rifles I've been able to dial in very good trajectories from the first few shots, I feel that getting the optic proofed, solid environmental data, a good G7 BC and leaving the crony at home help the process! I got my velocity by shooting at a zero 300 yrds and 590 yrds and tuning it till the trajectories were spot on, then shot out at the max travel of the optic which was just about to transonic and it was right on and has been right on no mater elevation or temp. I'm sure not all will go this easy but I sure was grateful to have a couple easy ones!

 

[Buffalobob]

Goodgrouper kept records for one or more of his rifles as the barrel deteriorated and found that the BC degraded as well.

It is a fairly well accepted belief with benchrest shooters that some barrels will shoot better in the wind than other barrels.

It is also fairly obvious that different configurations and diameters of barrels affect the structural stability of the bullet and if one compromises the structural stability then one may have compromised the BC.

There are several other barrel factors that will affect the BC of a bullet and those of us who shoot the extreme distances particularly with the bullet under discussion are well aware of some of them.

 

[Michael Eichele]

Michael,

Measure you scope bore ht, verify your zero, verify all environmental conditions, adjust for the scope error your scope has per click value, use the litz G7 BC and lastly, adjust your trajectory fit by adjusting the velocity in the calculator to get onto center mass at AT 1000yds - dont listen to your chrony. I say 1000yds, because this is the distance Bryan Litz averages his derived BC`s across and is a large enough distance to achieve a decent resolution. I say dont listen to your chrony, as its apparent to me after reading your previous posts, this is definately a source of a good portion of the error. My chrony also gave me headaches...

Let's say for a minute that your are correct and the only problem here is my chrony and my friend's Ohler, and my 2nd chrony. In one of my loads, I would have to adjust my velocity from 3242 to nearly 3460 to make the trajectory match closely using the Litz BC.

If this is really the case, I can live with that. I still think 3460 ish FPS in that rifle is not possible, however, IF this really is the case, I can accept that.

What I cant live with is if I use your method and if in reality is not the right way.

Why do I care if it is or isnt as long as the trajectory matches?

Because if I use a higher velocity than what reality is and a different BC, the calculated velocity at long range is different than when the lower velocity is used with another BC. As much as about 200FPS at 1000 yards. Even though the trajectories match closely.

Why do I care?

Because I need to know the real velocity at the impact range so I know if the bullet will expand or not.

Now I realize that chronies are not 100% perfect. I do however have a hard time believing that multiple chronies even some that are not the same brand and some of which Litz probably uses himself, are all off by 210-250 FPS at around 3300 FPS. In fact, I would venture to say that it is not possible that 3 chronies, 2 of which are not the same brand, are off by that much. I can buy 50 FPS but not 250 FPS.

In another load (338 300 grain Hybrid), I only have to adjust my velocity 13 FPS to make the trajectory match using the Litz BC. Now that I can live with, but not over 200 FPS. I might be able to buy it if every bullet worked out to 180-220 FPS but when one is 13 and another is 220FPS especially when I know that load is probably not capable of running that fast, I have a real hard time accepting that as a proper method.

Or maybe, a viable explanation is that an average BC value does not work when that bullet is fired at above or below average velocities. Regardless, I need to know which method is right because I need to know if the bullet will expand at the range I am wanting to shoot it at. Some here say use the BC value that fits and others say use the velocity value that fits regardless of chrony measuments.

Can anybody prove here that my or anybodies chronies are causing 200-250FPS errors? If you can show me that my chronies have a 200+ FPS margin of error, I will adopt this new 'adjust the velocity to fit' method.

I can agree that if used in the sun one day and the shade the next or if one photo eye is in the sun or the other is shaded, there can be pretty sizable inaccuracies. I am not talking about these. I am aware of them and avoid those scenarios. Again, show me how they are off by 13 for one load and 220 for the next and I will adopt this method.

 

[rscott5028]

Michael,

I agree with you that 3x chronies which correlate from 2x manufacturers is more certain than a single chrony.

I'm curious if the microphone method referenced in the following post by Groper would further corroborate the velocities reported by your chronies.

Posted by Groper:

Re: Let's argue about BC's
If you want to do a more accurate real world BC calculation, rather than do a drop test, provided you own a laptop computer

try this --> http://arxiv.org/ftp/physics/papers/0601/0601102.pdf

Follow the theory outlined in this paper, we are going to use the same theory for BC calculation rather than velocity calculation. We will shoot thru the chrony for the velocity.

Remember to allow 0.001 seconds for every foot your microphone is from the muzzle AND gong.

Make sure you allow for wind speed in your speed of sound, or simply wait for a no wind day.

Make sure you get the air temperature exact.

Shoot a gong at as much range as possible, the mic of your laptop will pic up the sound of a gong @ 1000yds.

Record each shot velocity and time of flight seperately. Once you have enough data, run several calculations to figure BC, and use the median result for your actual real world BC.

You can thank me with beer

 

[eddybo]

Bob, I know you have shot some extreme distances, and with the bullet that got this discussion started. What chances do you think you would have at hitting a target at 1500 yards using the .74 BC versus what you are using now to make long hits?

What velocity adjustments do you think you would need to make things come into line with the actual drops of your rifle using .74?

I am not asking what you think the true BC of the bullet is, just number works best for you to make long range shots.

Goodgrouper kept records for one or more of his rifles as the barrel deteriorated and found that the BC degraded as well.

It is a fairly well accepted belief with benchrest shooters that some barrels will shoot better in the wind than other barrels.

It is also fairly obvious that different configurations and diameters of barrels affect the structural stability of the bullet and if one compromises the structural stability then one may have compromised the BC.

There are several other barrel factors that will affect the BC of a bullet and those of us who shoot the extreme distances particularly with the bullet under discussion are well aware of some of them.

 

[Michael Eichele]

Michael,

I agree with you that 3x chronies which correlate from 2x manufacturers is more certain than a single chrony.

I'm curious if the microphone method referenced in the following post by Groper would further corroborate the velocities reported by your chronies.

I toyed around with that a couple of years ago. I did a search on google a couple years ago for sound and velocity (I dont remember the exact search key words). I found a case where there was a paint baller that was using Audacity in his apartment where he would fire ball against a peice of plywood and caluclate the difference between the firing and the ball's impact. I will admit I didnt give it nearly the attention that it deserved and would like to revisit this.

Along with that, a couple of years ago I had wrote some code for a program where you can start and stop a high speed clock. At multiple points need be. Used in conjunction with microphones and relays and a computer I/O module, 1000' of cat5 cable you could lay these mics out and connect them to the laptop, fire of the mics where the muzzle blast/sonic crack would activate the clock, stop it at 100 yards, start the 100 yard clock, stop the 100 at 200, start 200 and the 300 yard would stop the 200 and compare that against the 300. You could get 3 time of flight values over 300 yards. Actually you could do it for as far as one would want. The challenge is affording and managing muli thousands of feet of cat5 cable and 10-20 mics. I scrapped the project due to the cost of components and the quantities needed for a very accurate profile. I still have the working code and has been tested using simpler and cheaper methods of starting and stopping the clocks. I am half tempted to re-vist this system for the simple reason to verify what system is actually right.

Thanks for your input.



M