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BALLISTIC COEFFICIENTS/Twist Rates

 
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  #22  
Old 08-28-2010, 01:39 PM
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Location: Texas
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Very good information Lou and Bryan !!!!!

I had not thought about using the flat base bullets in over 50 years because I have all ways
liked to shoot as far as my ability and equiptment would allow and the only choice back before
computers was the boat tailed bullets.

All of my trajectories tables/ drop charts had to be fired back then to recent times because
listed BC were calculated differently from one bullet maker to the next and could not be
trusted.

But now that we have the computer it would be nice not to have to re zero for changes in altitude,
temperature,humidity and so forth buy actually shooting a group under these
conditions and have to modify your drop charts across the range to be used.

What I see as the main benefit of good BCs for each bullet fired in a particular rifle is
the ability to calculate a solution that will allow a shooter to correct for all known conditions
and be able to predict bullet impact at any reasonable distance.

I am not very computer literate but I do know that the better information you put in the
better you get out of it. so that's why I would like accurate BCs not high numbers that don't
work in anything well.

What's the saying- Close only works with horse shoes and hand grenades.

Please keep the debate going for the less informed members Like me.

Thanks
J E CUSTOM
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Last edited by J E Custom; 08-28-2010 at 07:50 PM.
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  #23  
Old 08-29-2010, 11:07 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by bsl135 View Post
(snip)
Am I correct in understanding your suggested way of 'predicting' a trajectory is to shoot your rifle and write down the drop?!?! Talk about Before Computers! Of course it's a good idea to verify your predicted drop just to be sure the theory matches reality, but don't forego the step of predicting the trajectory in the first place, it's quite useful! Plus you can't shoot in every condition (elevation, slant angle, wind, etc) so at some point you have to rely on a predictive model.
(snip)
However I also understand that to actually predict trajectories thru the transonic speeds for most bullets requires aerodynamic models that are not available (or would take years and $100K's - doplar radar - to develop for each round) AND would only be relevant for a particular rifle barrel... All this to predict trajectories beyond what most have the accuracy to hit a vital zone anyway... I don't think that's a 'better' way.
-Bryan
I'm not advocating trying to make a complete drop chart for each bullet/load/rifle. What I'm suggesting is that accuracy can be improved by doing shooting tests at the ranges you expect to shoot a particular rifle, then adjust the ballistics program to give a better match to real trajectories. It does help to have some instrumentation like a chronograph, a method of measuring air density, and to do the test in low/no wind conditions. The BC is a reasonable number to tweak to get the drop to match if you have good certainty of your velocity and air density, particularly since many programs already allow for multiple BC vs velocity inputs. For those programs which calculate spin drift the twist rate input to the program can be tweaked to match the shooting measurements of horizontal drift. Wind can certainly interfere with the tests, and can easily be larger effects than the errors that the shooting tests are attempting to correct. I use manufacturers BC's as a starting point and G7 values when they're available. I'm not trying to write a new ballistics program wich does not use BCs and there would be no available input data if I did. Should shooters attempt to do make these corrections? Only if they have the time and care to go to the effort.
At lest though they should make some shooting tests just to verify that their computations are close to reality. It's a little late after they shoot an elk in the leg and it hobbles off over a hill 800 yards away.

Shooting tests are a way to correct for situations such as the original poster was asking about, in that case why the spin rate appears to affect BCs. Few ballistics programs include the code to predict that effect, whatever the physical explanation may be. For most hunters getting hits close to the point of aim on first and subsequent shots are more of an objective than understanding the physics of how air affects bullets.

I agree that shooting though the transonic region is wrought with problems. Existing McCoy based ballistics programs and BCs are nearly useless for predicting transonic performance. The spin rate of the bullet as it goes transonic is rarely known at all and the programs like McGyro don't calculate the decay of the spin rate vs velocity or time. I am not recommending hunting or even target shooting beyond transonic range and velocities. It's interesting though that transonic shooting used to be common for the military. Many WW1 rifles had sights calibrated to 2000 yards or 2 km. They were used for volley fire on massed troops. It is possible to get pretty good accuracy shooting transonic. Few "long range" modern rifles however have spin rates or bullet designs which make it practical.

I'm in that 0.1% you mentiond who is interested in trying to understand the physics including shooting subsonic and shooting though the transonic range. I suppose this is the wrong forum to even bring it up. I do think though that every shooter who uses ballistics software should be aware that all programs are limited models and they need to understand what the limitations are. Used outside of those limits they can give very wrong answers. There are many small amplitude ballistic effects with few, if any, available programs even address. No, I don't expect BCs will go away for sports shooting. Both Sierra's multiple G1 BCs and Berger's publishing of G7 BCs are fairly successful methods for improving the predicted trajectories.

Bryan, I don't disagree with anything you said in your last post. You support Berger bullets and their objective is to sell bullets and make them useful to the shooting public. For that I think you and Berger are doing an excellent job. Even I don't think there would be a practical advantage in developing new ballistics programs and doing millimeter radar measurements to get drag functions for each projectile. It could only be cost effective if the equipment was used by many bullet manufacturers.

You've reminded me that the effort required and money spent would be better directed. Some of it no doubt toward developing a practical instrument to accurately measure the wind along a bullets trajectory. That would give a greater improvement in long range shooting accuracy than making new ballistics models possibly could even if they are perfect.

Last edited by LouBoyd; 08-29-2010 at 02:06 PM.
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  #24  
Old 08-29-2010, 07:14 PM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Lou,

We're on the same page for the most part. I only took issue with your treatment of BC's as a useful concept because you seemed to imply that BC's weren't 'good enough' for general use.

As far as 'tweaking' BC's to match observed trajectories, I have to raise a caution flag there as well.

There are many variables that can make it look like the BC is in error, when in fact it's something else. For example, a common one is scope reticle movement. If the program predicts 10 MOA of drop and you dial your scope 10 MOA and hit low, well, the BC must be off, right? The serious shooters here on LRH know better but many shooters are quick to question the BC because it's the most mysterious and (generally) the least understood variable. You can 'tweak' the BC to get it to make up for an out-of-calibration scope, but it won't really be right. You end up chasing your tail by defining multiple BC's to make up for the scope error, and in the mean time your predicted impact velocity, energy, wind drift, etc are all in error because you're feeding the program a false BC to force it to match the skewed drop values resulting from the out-of-calibration scope.

The out-of-calibration scope is just one example of a common error source but there are many others that tempt people to 'tweak' BC's to match their observed trajectories.

Even having said all that, I acknowledge that there are still some cases where a certain rifle with a certain barrel will fire a bullet in a way that slightly affects the effective BC that bullet flies with. Variations in BC tend to be small for accurate rifles. If too much yaw is induced, or too much surface scaring, etc, the rifle will probably not be accurate by our standards.

As to the test results from the original poster, I can't remember the details now, but when questioned, I seem to remember some missing information like some key atmospherics, or velocity measurement, or something that the measured BC's were based on. No disrespect to the tester, but truly accurate BC testing is HARD. Even if your test procedure is sound, you can easily have instrumentation error that you don't know about. Is your Kestrel calibrated? How do you know your chronograph is accurate? What about your laser rangefinder? When you test BC's, every single variable has to be known to a high degree of certainty or the error will compound on the final result.

I do think that unique drag functions are useful for those projectiles that remain comfortably stable thru transonic. If the bullet exhibits low pitch/yaw, then it's trajectory can be predicted quite well at all speeds, but most bullets fly unpredictably at transonic, making prediction a futile effort.

Thanks for the healthy exchange Lou. Always good to investigate and discuss alternate approaches.

Take care,
-Bryan
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Author of: Applied Ballistics for Long Range Shooting
And: Accuracy and Precision for Long Range Shooting

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  #25  
Old 08-29-2010, 10:48 PM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Good info bryan, id have to agree 100% with your comments on "tweaking" the BC to match known drops.

My last scope tracking was unknowingly out by 5%. So instead of dialing 8mils i was actually dialing 8.4mils. I ended up chasing my tail for ages tweaking velocities and BC`s to match the trajectory i was getting, never getting it 100% correct until i finally found the problem. Same goes for the Zero, it has to be 100% spot on and remeber to rezero it if you tweak your load in any way or shoot in different weather conditions.

Bryan, i found your ballistic testing and measured/calculated G7 BCs be of greatest help in this regard also. Using your G7 BCs i tweak my velocities now to line up my trajectories with known drops at 1000yds and it gets me damn close, certainly less than 1 click at this distance which is certainly closer than using my chronographed velocities and tweaking the BC. Obviously my chrony isnt accurate enough.
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  #26  
Old 08-29-2010, 11:19 PM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by groper View Post
Good info bryan, id have to agree 100% with your comments on "tweaking" the BC to match known drops.

My last scope tracking was unknowingly out by 5%. So instead of dialing 8mils i was actually dialing 8.4mils. I ended up chasing my tail for ages tweaking velocities and BC`s to match the trajectory i was getting, never getting it 100% correct until i finally found the problem. Same goes for the Zero, it has to be 100% spot on and remeber to rezero it if you tweak your load in any way or shoot in different weather conditions.

Bryan, i found your ballistic testing and measured/calculated G7 BCs be of greatest help in this regard also. Using your G7 BCs i tweak my velocities now to line up my trajectories with known drops at 1000yds and it gets me damn close, certainly less than 1 click at this distance which is certainly closer than using my chronographed velocities and tweaking the BC. Obviously my chrony isnt accurate enough.
Good point Groper,
Lets talk "Tweaking"............
Say you have a scope that is out 5%. What's the best way to compensate for this error given that shooting under a hunting situation, the distance, pressure, temperature etc will always be different?

I have two scopes that have about 7% error in the elevation adjustment. What I do is measure the velocity, then calculate the real drop required at about the maximum range that I intend to hunt using the best available B.C. numbers that I can find including Bryan's G7.

I then shoot at that distance and measure the error on the target. I then adjust the dial cap to read the calculated dial up and then shoots at closer ranges. The closer you are to the target the less the error matters.

ie, Say I am out by 2 MOA at 800 yards. I move the turret 2 MOA and lock it in position.

I may then be 1 MOA out at 400 yards and 2 MOA out at 100 yards but that is better than being 2 MOA out at 800 yards. If you follow my drift.
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Last edited by Topshot; 08-29-2010 at 11:41 PM.
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  #27  
Old 08-30-2010, 12:35 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Well luckily my ballistic calculation software allows me to input a "custom click value" so i simply get it to output the elevation in "0.105 milrad clicks" instead of "0.1 milrad clicks" like its supposed to be.

Failing that capability, i would simply adjust the total elevation required by 5% and round that number to the nearest click. In your case 7% adjustment.

So if your calc is telling you you need 35MOA for the shot and your scope is out 7% over for instance;

multiply 35MOAx1.07= 37.45MOA = rounded dial ON THE SCOPE 37.5MOA.

If its out by 7% under;
divide 35MOA/1.07 = 32.71MOA = rounded dial ON THE SCOPE 32.75MOA.


DO NOT tweak the G7 BCs that bryan has published, they are closer than 1% accurate, they do not need tweaking. Its the velocity and scope where the problems lie.
Reverse calculate the ACTUAL drops from the elevation you dialed on your scope taking into account 7% error.
TWEAK YOUR VELOCITY, to match the ACTUAL drops at as much distance as you can, No less than 800yds preferably but the more the better, 1000yds is ok.

IF you dont have a tested verified G7 BC from bryan, then things get a whole lot more difficult and the best way forward would be calculate your own BC from shooting thru 2 chronies as far apart as possible, 1 at the muzzle and 1 at 500yds would be ok.

Last edited by groper; 08-30-2010 at 12:45 AM.
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