Let's argue about BC's

[ZEEK]

I have a question for the experts , why does the same 350smk from two different 375 rifles have different Bcs although the velocities and twists (12 and 10) are different?

If I understand history correctly, the BC was developed as a way to address the fact that the drag force is different on every type of projectile. So to simplify things and not have to make measurements of drag deceleation for every type of projectile, the BC was proposed as a method to scale the drag deceleation from one projectile, let's call it the "standard" projectile, to another projectile, our bullet if interest. So we see the BC is all about drag.

Lots of things effect drag, the spin dampening moment is one of them. One of the factors of the spin dampening moment is angular velocity (how fast the bullet is spinning). So two projectiles with the same velocity, but different spin rates--the faster spinning projectile experiences more drag. The second thing that could play into this is the yaw angle. When a bullet leaves the barrel, the axis-of-rotation of the bullet does not exactly correspond with the axis of the barrel (due to barrel vibration and end effect from the end of the barrel), these differences in yaw angle will also effect bullet drag as the drag coefficient is a function of the yaw angle. The final thing is that the higher spin rate may also allow the yaw angle to more quickly reach the yaw of response (or equilibrium yaw).

So we see a lot of factors come into play and can effect a bullet's BC. In the end, if you want good accurate data for yor rifle--go to te field and make good measurements.

 

[groper]

Eddy,
Exbal cant handle G7 can it? But will it handle a multi G1 set?

this relates to exactly what i said earlier... when your shooting allen magnums with extremely high velocity, you will need to use a much higher G1 BC for the shorter ranges or first part of the trajectory than BL publishes... as i said, G1 BC is very velocity dependent - so very high velocity VLD bullets will exhibit much higher G1 BC`s whilst their speed is high... However, you will find that toward the end of the trajectory, at very long range when theyve had a chance to slow down, the bullet will impact much lower than predicted... The only way to combat this, is to input a lower BC for the slower part of its flight regime by way of a velocity bracketed G1 set like sierra publishes.

Whats interesting is that BL would have certainly given Paul a velocity bracketed set, not just a single G1 BC - Paul has chosen not to publish this it seems so we cant see how fast the G1 BC of his 7mm 200gr decays with velocity... He would have also given him a single G7 BC - because thats what Bryan does.

Remember, BL is giving us AVERAGE BC`s, for the flight regime in which he tested them. If he tested them from a very fast 7mm caliber such as a long barrelled allen mag or STW etc. then this would make the G1 numbers look much better on paper. If he tested them from a 7mm-08 or similar, then the G1 numbers would look much lower...

This is why BL so strongly believes in adopting the G7 system- because it wouldnt suffer from the same inconsistencies and require so many caveats when quoting the G1 BC - predominately - how fast the bullet is going, and how fast the BC decays from that point OR how much better it gets if your lucky enough to be able to shoot it very fast. G1 sux...

The G7 number also gets higher with more velocity, but by MUCH less than the G1. A single average G7 number, will give 99% of users a very good trajectory fit when you give your calc correct data on everything else (which isnt hard).

Lastly, if your using a Litz G7 number, and you dont have some exotic extremely fast or slow caliber, and your still having problems with your trajectory fit - then you pretty much are guaranteed to have errors in your other inputs or your chrony could be telling lies... Mr Litz is a professional, and his methods are very accurate, but he is not the aberdeen proving ground either so he cant give us BC`s to suit everyone, only 99% of us...

 

[eddybo]

Exbal handles multi BCs. Shooter for my droid phone, quick target and the program on my ipod all accept g7 Bcs....at least I think the program on my ipod does.

I guess maybe if you get way out there the BC would drop low enough, I do not have a place to shoot further than a mile.

I would guess that even at the length of my range in the back yard, 800 yards the difference between a .74 and a .9 BC should be apparent.

I have had some sucess with G7 BCs, some failures. I do not find it be the end all of LR accuracy, especially since there is no way to adjust the G7 BC in shooter, that I can find. In fairness, I have really only done enough shooting with that program using the 162 a-max. I find that the further I shoot the higher on the targets my shots hit. Like I said I am sure it is my mistake somewhere. I do not care enough about the issue to even refute any argument regarding BC. My only argument is that none of the BCs are infallablle, and you have to shoot the rifles.

I used to think, as I am sure many still do that all you gotta do is plug in a BC and a velocity and you are on target to infinity. Does not work that way even with the G7 BCs from my very limited experiance with them. You have to shoot these things and confirm the "educated guess" these ballistic calculators make.

 

[jwp475]

Exbal 10.0 for the desk top computer handels G-7 drag BC's

 

[groper]

I would guess that even at the length of my range in the back yard, 800 yards the difference between a .74 and a .9 BC should be apparent.

Assuming you have all the variables correct besides the BC, its only 3 clicks difference @ 800yds on your scope assuming a 3200fps velocity....

If your at 5000ft above sea level, its only 2 clicks difference.

If you then use 3250fps to the velocity and .74 BC, your less than 1/4MOA click difference from the 0.9 BC and 3200fps.

This is without any other errors creeping in...

See how fast things can go wrong?

Are you saying that youve shot your 7mm AM to 1 mile, and used a 0.9 BC, and your trajectory was too high??? If so, then you have HUGE problems with your calculations and your trajectory fit... If not, can you explain it better?

 

[jwp475]

I used to think, as I am sure many still do that all you gotta do is plug in a BC and a velocity and you are on target to infinity. Does not work that way even with the G7 BCs from my very limited experiance with them. You have to shoot these things and confirm the "educated guess" these ballistic calculators make.

I had trouble with my drops not matching Exbal. I tried different BC's trying to make it work, but I found that I could make it match at certain distances by tweaking the BC not all distances.
So I went back to square one and started check every variable, score height, zero, reticle level and even re chrono'ed my load.

When I originally chronographed my load the day was very sunny and bright. I average velocity with the 300 SMK was 2820. When I re-chronograhed my load I got 2791.

Using 2791 FPS my drops now matched with the published BC. 2790 was the corrected velocity in Exbal when I used the trajectory validation feature.

I believe that when a carefully measured BC is not match ones actual trajectory it is because there is an input error that one has missed

Makes life much easier when you live near sea level and are planing a hunt at 7,000 or 10,000 feet elevation if the data from your ballistics program can accurately predict your settings without needing to re shoot at the varied distances to confirm

 

[MSU Marksman]

One thing that comes to mind;

When these tests were being preformed, were they also being preformed against other bullets at the same time? Were the variations in the Wildcat bullets similar to variations in other bullets as well?

 

[Kevin Thomas]

I have a question for the experts , why does the same 350smk from two different 375 rifles have different Bcs although the velocities and twists (12 and 10) are different?

Need a bit more information, but I can offer a few comments with what's here.

1) you don't mention how much the BC changed. What I've seen many shooters freak out about really amounts to nothing more than the normal standard deviation seen within a string of shots when testing BC. Yes, they vary, sometimes substantially from shot to shot.

2) were these bullets from the same box, or lot? Again, they can vary substantially from one production run to the next. Even more so, if we're talking about spitzers, given the trouble with achieving a constant blend and tip shape.

3) Yes, atmosherics most definately DO play a role, and a fairly major one at that, when measuring BC. What you're dealing with is raw data, or an accurate BC for that time and place, which might be what you want. The manufacturers published BCs should all (and as far as I know, are) corrected back to ICAO or Std Metro corrections. This again allows a comparison for any given bullet against any other. In inputting altitude or pressure parameters in a ballistics program, you're in effect correcting this back to your conditions.

4) Twist really makes very little difference here, until you reach the point at which a bullet verges on becoming unstable, then it makes a big time difference. I ran a series of tests with Bill McDonald on this that are in the 4th Edition Sierra manual, if I recall correctly. The plots are shown for several bullets in a range of five or six different twists for each caliber. They began with very high stability factors (SG) and proceeded on down to the point where they were nearing a SG of 1, or actually dipping below. The shots were plotted, and are there for all to see. I'll point out that the tests were run at 200 yards (which was what we had available) and that these figures may become more pronounced at geater distances.

5) There's nothing wrong or antiquated about the G1 drag model. It was developed at precisely the same time (and by the same fellow) who gave us the G7. And the G5, G6, GL, etc., Mr. E.D. Lowrey of Olin-Winchester. The problem (as we see it today) is that the manufacturers standardized on this one drag model for everything, when it's not well suited to many. It IS the ideal drag model for some bullets, and dsitinctly not the right one for today's VLD style projectiles. The G7 is, likewise, not the ideal projectile for many sporting bullets, such as Spitzer flat based designs, round nose designs or a host of others. Bryan's piqued awareness of the fact that there's a better model for the VLDs than the G1, and that's what started the ball rolling. But to be precise, yes, there are other drag models out there and they'd be better used for the designs I mentioned. Getting complicated yet?

6) velocity is an important factor, especially as range increases. Basically, we're seeing the actual performance diverge from the drag model's limitations in how they predict the downrange results. Using a VLD style bullet and the G1 drag model, the results will split pretty quickly and become more pronounced as the distance increases. Using the correct drag model, and those variations become much less, and ideally, stay spot on.

7) Lastly, the use of multiple BCs is generally an attempt to account for this discrepancy, and make the results "fit" the trajectory we actually see on a shot fired in the field. It's a patch, and helps. The fix, is to use the correct drag model.

 

[eddybo]

Groper

3 clicks is over 6 inches at 800, sounds readily apparent to me. I do box tests on my long range scopes and am usually shooting through a chronograph off of a cement table that is indoors. I am at 200ft altitude and am shooting at fixed targets at known distances. I am pretty sure I am correct as to the distances to my targets as I have ranged back to my shooting house many times over the last 3 years confirming those ranges. I have adjusted for sight height but am not aware of a way to really correct for base angle.

I have not shot the 7mm 200gr wildcat at all although I have shot the 300gr 338 and 169gr 277s and a whole bunch of the 6mm 65gr wildcats with great sucess.

I have shot a different bullet that I use a exbal BC of .89. Bryan modeled that bullet at .84ish which he sent to me confidentially, but he later shot and announced a .65ish BC for that bullet. I am saying that if you plug .65 or whatever number it was into exbal I was over the top of a 4x4 target at 1760. The range where I shoot to a mile is less than 6 miles from my home. I usually shoot through a chronograph at home confirming my drops out to 800 yards. Then I load up my rifle and ammo and go shoot long range. (yea I am referring to the 265gr HAT 338s)

My first shots after I move to the longer range are usually at 1400 and if everything is working I drive back up to the mile mark to shoot, stopping to shoot 1K at a smaller berm on the way. I have actually shot this bullet a good deal further than a mile, but only to a mile in a semi controlled enviroment. I use a stepped BC and have not had any problems matching my actual drops to what exbal predicts. The place where I shoot one mile is on a slight hill shooting down into a river bottom, but I have been unable to get a reading as to the inclination it is so small. I cannot figure out what I am doing wrong. I have given up trying to figure it out and continue to use the .89 stepped G1 BC.

It has worked well enough to kill elk at 1200, several pigs further than 1200 and a yote at 1620. Thus my position that BCs do not mean anything. Actually shoot the gun, being as correct as you can. Find a number that will allow you to make hits as far as your can shoot. BC is nothing to me other than a number that I manipulate to make my actual trajectory match exbals outputs.

I have no explanation why some manufacturers BCs work pretty close, some not so close, point is, it does not matter if you actually shoot enough to know what number actually works. BCs are nothing but a selling point, and a starting point to figure a real "working" BC.

I have two 6mms that I shoot long range with, same chambering, both barrels arrived from kreiger on the same day. Using the same lot numbers of brass, powder and bullets actully from the same containers, I can load for both rifles. I can then chrono them and shoot them long range with scopes that I have box tested. Using the same profile in exbal to change only the respective velocities less than 100 fps I have a had an 18 inch differential at impact at 1400 yards, using exbals adjustments for the respective loads. I do not know where the errors arise, nor do I at this point care. I only know that if you want precise drops you are going to have to shoot the rifle.

I have found several manufacturers BCs to be close enough for most purposes in the rifles I have shot them in. I have shot the same bullets in other rifles and had to make substantial adjustments to them. (caveat many times this is in rifles I build for other guys and do not have then guns long enough to really wring them out) Within 1K most any old BC is going to get you close, but you are going to have to make some changes to BC to get them to effectively hit targets at longer ranges.

Sometimes when you make these changes you are going to create small errors at shorter ranges. I would rather have small errors of say plus or minus .1 or .2 MOA at shorter ranges than an error of plus or minus .3 or .4 at 1500 yards. Same principle as actually zeroing a scope at 800 yards, then dialing back to my 100 yard zero giving me a bullet hole sized error at 100. You have more margin for error at shorter ranges. After I have done all of my shooting to confirm drops I try and move all of the errors in scope adjustment and BC adjustment to closer range. You are absolutely correct that small errors can induce big changes, that is why I do this as a last step.

I have nothing but antecdotal evidence gatherd from my purely unscientific methods to bolster my statement that BCs do not mean much. I shoot with guys who rely solely on BCs optained from manufacturers and someone else's work from the internet. They usually wonder why my guns shoot better at long range than their gun does. I like to let them beleive that I am a better shooter which is not the case. If they would do the work they could probably do better than my efforts.

Arguing about BCs is purely an academic exercise, but has very little real bearing on where a bullet is going to hit (within a certain error range (( I do not know from a scientific degree what an expected error range would be for a G1 or G7 BC. Is it plus or minus 3% 5% who knows?). I suspect that Bryan is right as to what actual BCs are, after all his BCs for Berger bullets are usually prettty close to working. Maybe all the manufacturers BCs are so inflated that code writers for ballistics programs have had to make changes to make their programs work. (being sarcastic here) I dunno what the answers are other than to shoot and shoot some more. Even when I am a long range hunt I am always hunting something to shoot to confirm my drops. On a four day hunt I might shoot 12 times even if I never shoot at an animal. Changes in wind sunlight humidity etc. always have us pulling over the truck or hiking to a big meadow so I can shoot the gun. Luckily the guy that usually guides me when I go out west is very understanding....and he gets a kick outta busting rocks etc at long range.

One thing for certain I have surely enjoyed the civil tone that is present here at LRH and hope to continue discussing this issue with you, and hearing the ideas of others. On some sites this would have already degraded to a aname calling match, not that I do not enjoy such antics at times.

Assuming you have all the variables correct besides the BC, its only 3 clicks difference @ 800yds on your scope assuming a 3200fps velocity....

If your at 5000ft above sea level, its only 2 clicks difference.

If you then use 3250fps to the velocity and .74 BC, your less than 1/4MOA click difference from the 0.9 BC and 3200fps.

This is without any other errors creeping in...

See how fast things can go wrong?

Are you saying that youve shot your 7mm AM to 1 mile, and used a 0.9 BC, and your trajectory was too high??? If so, then you have HUGE problems with your calculations and your trajectory fit... If not, can you explain it better?

 

[Jon A]

eddybo, when you box test the scopes how big a box do you make? I'm just curious as click value being off is a big reason people are often off at extremely long range and most don't test for it sufficiently.

 

[eddybo]

10 MOA at 100, but they are all elevation as I only have 7 MOA below my 100 yard zero, then 10 right and left. That was the way I was taught should I be using more? The scope being used in particular is a NXS 8-32. Come to think about it the scope was sent back to nightforce at some time for a failure to hold zero, but I cannot recall if it was before or after I was doing that shooting, so anything is possible. I guess that if the shooting was after the repair that the click values changed. I wonder sometimes how much the slant in the base could have to do with the anomaly.

 

[Jon A]

Yeah, you really want to do it for at least 10 mils, 35, 40 MOA or so. This will expose a small error you won't notice otherwise. If you think about it, a +/- 1% or 2% tolerance means out at distances you're inputting 150-200 clicks, you can be off between scopes 4 to 8 clicks and still be within tolerance, even if the two scopes are the same brand and model.

There are ways to do it without shooting as well if you don't have targets big enough.

 

[ericpetritz]

Very interesting thread! Many good points made and it is still civil, good stuff. Jon A---some very good points. There isn't much "art" involved in predicting accurate drops. If you have accurate inputs, drop predictions will very closely match actual drops. This is sometimes easier said than done. Some common input errors include inaccurate chronographs, inaccurate scope adjustments (i.e., a 1/4 MOA adjustment doesn't actually move the point of impact 1/4 MOA), inaccurate atmospheric conditions or entering them into the software incorrectly, inaccurate zero distance, inaccurate range finders, incorrect scope height, etc. On the other hand, marksmanship, reading the wind, etc are all about art and practice!

In my experience Bryan's BC are very accurate, both the G1's and the G7's, although you have to splice together G1's into velocity dependent bands with VLD's to minimize the error. Not saying he can't make a mistake, but I would look to other errors in the inputs before assuming his BC's are incorrect.

 

[groper]

Eddy, my NXS 5.5-22x56 has +2.5% error - which means when i dial 40clicks, im really getting 41 on the target... Ive had other scopes with upto 5% error like this.

I shoot a group @ 100, dial 100clicks up, shoot another group then measure on the target group 1 center to group 2 center. I dont bother doing the same with windage because the most wind ive ever had to dial is about 20 clicks and a few percent of this is gonna be within 1 click anyway...

I use a "custom click value" in my ballistics calculator to take this error out in my trajectory calculation- which becomes (0.1mrad*1.025) clicks. For this scope.

Eddy, have you tried adjusting your velocity instead of the BC? I dont use my chrony anymore in this regard, because IT was the source of half my problems... If you have a bullet that Litz has tested, use the exact Litz G7 derived BC, make sure all other variables are correct, including your dead on zero @ 100 for that load, ignore your chrony and adjust the VELOCITY in your calc, and see how your trajectory fits then- i bet it you will hit everything at all distances out to 1 mile...