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BALLISTIC COEFFICIENTS/Twist Rates

 
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  #15  
Old 08-24-2010, 04:51 PM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Kevin,
Always interests me how B.C.'s are derived.
When you did the testing. How was the B.C. calculated? Time of flight with multiple chronographs?
Need good equipment with such a short T.O.F. hey.

Any advice on the best method for a mug shooter could work out his own B.C. results.
I have been looking at a couple of projectiles that have a listed B.C. far lower than they should just looking at the bullet and would like to run a few tests on them.
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  #16  
Old 08-25-2010, 07:51 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Topshot,

In a nutshell, it's nothing more than firing the bullet in question over a measured distance and determining the drag deceleration over that span. The rate at which the bullet sheds (or retains) its velocity is compared against a standard bullet for whichever drag model you're using. In most cases in the industry, this will be the well known G1 model. Thanks to Bryan's efforts, and the support thrown to his ideas by Berger, we're now seeing the G7 model (far better suited to many of today's very streamlined boattail bullets) used on a regular basis. Using a better suited model to derive a BC results in notably more accurate long range predictions in computer programs and the like. As I said, that's the Reader's Digest version. There's a range of considerations that goes into this process, including the distance over which the measurments are taken, velocity ranges the bullets are fired, and as we've mentioned, sometimes twist rates involved. We would normally shoot 50-60 rounds at various velocites, going from extremely high velocity, to loads that were on the verge of going transonic within the span we were measuring. You see some interesting results. And, just like velocity, BCs will show Highs, Lows, Extreme Spreads and Standard Deviations within such testing. The BCs that are eventually published by the maker are roughly the average of this testing.

The other way of doing this is the method that Lapua now uses; Doppler radar tracking of bullets. This results in absolute "true" data, and allows the ballistician to see what the bullet is doing throughout its flight, not just a snapshot of it as it passes the two known reference points (skyscreens) in the straight TOF method. While the data is far better, as you can imagine, the equipment isn't available to the average reloader. The units themselves cost several million dollars and an extensive support team to operate. I've been priviliged to shoot data this way at Yuma Proving Grounds, but outside of military facilities, there's not many private concerns that could ever fund this sort of thing.

An abbreviated version here, but I hope it helps!

Kevin Thomas
Lapua USA
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  #17  
Old 08-26-2010, 02:39 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Thanks Kevin,
Good explanation. Not much hope then of the average mug shooter doing his own tests and coming up with any useful results?

If you just wanted to compare two different bullets of the same weight out of the same rifle. How would you do it?
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  #18  
Old 08-28-2010, 08:28 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by Kevin Thomas View Post
Topshot,

In a nutshell, it's nothing more than firing the bullet in question over a measured distance and determining the drag deceleration over that span. The rate at which the bullet sheds (or retains) its velocity is compared against a standard bullet for whichever drag model you're using. In most cases in the industry, this will be the well known G1 model. Thanks to Bryan's efforts, and the support thrown to his ideas by Berger, we're now seeing the G7 model (far better suited to many of today's very streamlined boattail bullets) used on a regular basis. Using a better suited model to derive a BC results in notably more accurate long range predictions in computer programs and the like. As I said, that's the Reader's Digest version. There's a range of considerations that goes into this process, including the distance over which the measurments are taken, velocity ranges the bullets are fired, and as we've mentioned, sometimes twist rates involved. We would normally shoot 50-60 rounds at various velocites, going from extremely high velocity, to loads that were on the verge of going transonic within the span we were measuring. You see some interesting results. And, just like velocity, BCs will show Highs, Lows, Extreme Spreads and Standard Deviations within such testing. The BCs that are eventually published by the maker are roughly the average of this testing.

The other way of doing this is the method that Lapua now uses; Doppler radar tracking of bullets. This results in absolute "true" data, and allows the ballistician to see what the bullet is doing throughout its flight, not just a snapshot of it as it passes the two known reference points (skyscreens) in the straight TOF method. While the data is far better, as you can imagine, the equipment isn't available to the average reloader. The units themselves cost several million dollars and an extensive support team to operate. I've been priviliged to shoot data this way at Yuma Proving Grounds, but outside of military facilities, there's not many private concerns that could ever fund this sort of thing.

An abbreviated version here, but I hope it helps!

Kevin Thomas
Lapua USA
Thanks Kevin !!!!

Has there been any attempt to change the standard model from G1 to G7 in the bullet industry ?

I realize that the numbers of the G1 BC look better but they are not accurate enough for the
long range shooter and in order to have a accurate drop table and BC we have to shoot
over the course that we intend to hunt to be accurate.

Also is there a more accurate model or should the models be Bullet type specific (Spitzer flat
base, Boat tail match, VLD and so forth). After all there are not that many different styles of
bullets an accuracy of numbers is what we need.

The long range shooter is all about accuracy in every aspect and have forced bullet makers to
improve there bullets so why not improve the system for comparing them and predicting there
performance more accuratly.

J E CUSTOM
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Old 08-28-2010, 08:50 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Has there been any attempt to change the standard model from G1 to G7 in the bullet industry ?
I've written each of the major bullet companies, asking them to consider changing the standard reference to G7 BC's. So far no takers. I can see their point that it would cause confusion with the 90% of shooters who barely know what BC is to begin with. It all that confusion worth it to satisfy the top 10% of technical shooters who understand? I think the paradigm has to change and the confusion is a growing pain, but I also understand why their reluctant. There's also the marketing aspect of surrendering a higher (looking) BC for a lower one.

It used to be hard to find a ballistics program that could use G7 BC's. However all the new programs coming out have G7. I consider that a good sign that we're moving in the right direction but it's going to take time.

It would be possible to have a different standard reference projectile for all bullet types (G1, G2, G5, G7, G8, etc) all have their place, but some of them are quite similar to each other. The difference between G7 and G5 is very difficult to resolve. I chose G7 as a standard because it fit long range bullets best overall, and long range bullets (long ogives and boat tails) are the types of bullets we care most about having accurate numbers for.

I was turned down when I suggested to the other companies that we should move to a different standard. Maybe over time, if their contacted by enough customers with the same request, it just might happen.

Good shooting,
-Bryan

PS. I should add that there are several small bullet companies (other than Berger) who are adopting the G7 standard including:
http://www.bibullets.com/new-products/
and
http://precisionballisticsllc.com/default.aspx
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  #20  
Old 08-28-2010, 09:32 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by J E Custom View Post
Thanks Kevin !!!!

Has there been any attempt to change the standard model from G1 to G7 in the bullet industry ?
(sni)
The long range shooter is all about accuracy in every aspect and have forced bullet makers to
improve there bullets so why not improve the system for comparing them and predicting there
performance more accuratly.

J E CUSTOM
As soon as you apply any "G coefficient" to a bullet reducing the information about the bullet to a single number (or even a few numbers) you have thrown away whatever useful information has been gathered about a specific bullet though testing. It's sort of like fitting clothing with the S M L XL XXL
designations but ignoring the variations of individual body parts.

G7 numbers are not inherently better than G1 numbers. They simply are a better but still limited model for slender boatail bullets typically used for long range shooting. G1 coefficients may be better for the bullets used in 100 yard benchrest and for small game hunting. Most 22LR bullets fit the G1 profile pretty well as do most bullets used for black powder rifles.

G1 and G7 coefficients are a handy way to compare bullets, and they are a useful marketing tool, but both are a detriment to precise ballistic calculations. What's is needed for precise ballistic calculations is to throw out all ballistic bullet models and simply measure (or calculate if there is sufficiently capable software developed) the actual drag curves for a standard atmosphere for each model of bullet made by each manufacturer in each caliber. All of that information would not fit in a practical size book, but it would probably fit on a single DVD ROM, or at least one from each manufacturer. The data would need to include a wide velocity range. It also needs to include stability information and the rate of spin decay vs velocity. There is presently no reliable method of calculating downrange bullet stability for a given bullet and atmosphere and determining it from shooting tests is very difficult. Existing published ballistic data is unreliable at transonic and subsonic velocities for any manufacturer that I'm aware of. Ballistic programs give answers you can't have any faith in it even when multiple BCs are given. Typical multi-BC data gives the lowest velocity BC number as covering "under 1800 fps" or similar.

What should a shooter do if they want an accurate drop chart for long range shooting with todays available data? The only answer I'm aware is not to rely on published BCs other than as a starting point. The only method is to use your own shooting tests instead. If you measure drop and lateral drift vs distance for several distinces over the range you shoot using your rifles and your loads along with measurements of the air density you can generate pseudo BCs which will give fairly accurate prediction of bullet drop and spin deflection. The existing ballistics programs are reasonably good for interpolation between the measured points and for adjusting for different air density. It's a lot of work and it will still have errors. Most ballistic programs have no model for handling bullet yaw, precession, and dampening so those errors will remain (and be more apparent) as air density changes from the conditions which existed when the shooting tests were made. The ballistic software simply has no code to account for those effects. Some software does exist (six degree of freedom models), but its usually impractical to obtain precise enough data in the input conditions to make the calculations more useful than the commonly used "McCoy" model.

I consider the term BC to mean "Before Computers" more than "Ballistic Coefficients". They were useful in the first half of the 19th century when ballistic calculations were done by hand using books of log and trig tables. Error correction (in the math) was handled by having multiple people do the calculations and comparing the results. At that time using a few simple ballistic bullet models made a lot of sense. But mathematical computation is no longer a limitation. Complete ballistic modeling can be done using a $300 PC from WalMart in less than a second. What's missing is the drag vs velocity measurements and stability data for each bullet from each manufacturer. Yes, that's a lot of data. Few manufacturers (any?) have it to publish.

A few manufacturers are giving G7 instead of G1 coefficients for bullets where the G7 is a better model in the velocity range most hunters and target shooters use. As long as people belive that "better" BCs are desirable bullet manufacturers will continue to publish them. Shooters seeking the highest accuracy are still free to ignore the published BCs.

Last edited by LouBoyd; 08-28-2010 at 11:19 AM.
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  #21  
Old 08-28-2010, 10:01 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Lou,

I understand the approach your advocating, but disagree that it's the right approach for modeling ballistic trajectories for 99.9% of shooters.

To start, the difference between a trajectory modeled with BC referenced to the closest standard, and a trajectory modeled with the bullets unique drag curve will be less than 1 click thru the bullets supersonic range (which is where most shooters, and especially hunters are concerned).

When the bullet slows into the transonic speed range, stability becomes questionable and the trajectory becomes unpredictable as you said. However custom drag curves for each bullet will not solve that problem. You have atmospherics affecting stability in a way that indirectly affects the drag (induced drag) of the bullet, as well as the effects of different rifling patterns (depth, smoothness, # of riflings, etc become important when dealing with transonic stability). In other words, transonic flight is determined by way more than drag. Heck, most bullets just tumble at this speed, making any efforts to predict their trajectories impossible and irrelevant.

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.

Using a good ballistic solver with accurate input data (including an accurate G7 BC) will result in trajectory predictions that are accurate within 1 click thru the supersonic range of the bullets flight. Below supersonic speeds, I agree, all bets are off. 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.

Traditional BC's as we use them are very useful not just for comparing bullets to each other, but also for computing very accurate trajectories for the relevant flight envelope of the bullet.

-Bryan
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