Any hints for finding bullet BC (Ballistic Coefficient) ?

S

Sevens

New member
Something I have enjoyed a lot this season is setting up at the rifle range with a sandbag and any of a number of handguns... and shooting at my steel plates. I often hang an 8-inch gong at 100 yards and this is not all too tough to hit. Making hits at 200 and 300 yards is far more difficult but it is a -LOT- of fun and my longest hits have been at the limit of the range at my club... ringing a 12-inch double-plate hanging at 375 yards.

I have a little app for my phone that does a fantastic job at predicting my hold over and bullet drop and impact velocity... but to do the best estimates that it can do, it wants to know the BC of the bullet.

In some looking here and there, it seems to me (unless I am mistaken?) that the BC assigned to the bullet is pretty much assigned by the bullet manufacturer and even then, it's kind of their opinion and it could be argued.

Finding a BC from a bullet manufacturer isn't all too tough when you are talking about center fire rifle, but it becomes quite a bit more difficult when you are lobbing handgun bullets at great distances.

Does anyone have any tips or handy web links for a collection or database chock full of bullet BC's, so I could compare my slugs to something similar and borrow the BC of that bullet?
 
Even with rifles a significant difference in bullet drop caused by BC's doesn't show up until you start shooting at 300 yards or farther, usually much farther. I could be wrong, I'm more of a rifle shooter, but I just don't see it making a huge difference with handguns.

In my 308, the difference between a BC of .429 and .415 is around 1/4" less bullet drop at 300 yards.

The energy numbers can be greatly improved at ranges as close as 100 yards with the same bullet weights, but much better BC's. But bullet drop, not as much until you really get out there.
 
I hear what you are saying -- but if we consider that rifle bullets are (generally) streamlined and well designed for long flight and handgun bullets are fat, short & stocky, and not at all designed for long distance flight... would we not come to the conclusion that BC has a much more pronounced effect on the handgun slugs?

I suppose my most satisfying hits at distance doing this kind of shooting might be hits with my PPC customized old Smith & Wesson pre-Model 10. Running 158gr plated SWC slugs around 800 fps, hitting a 6-inch plate hung at 200 yards took an estimated twelve and a half feet hold-over. But when you connect, you have to laugh. :D
 
BC = mass of bullet in kg or lb. / measured cross section (diameter) of projectile in m or in X Coefficient of form.
The Coefficient of form (i) can be derived by 6 methods and applied differently depending on the trajectory models used: G Model, Bugless/Coxe; 3 Sky Screen; 4 Sky Screen; Target Zeroing; Doppler radar.
OR.
BC = The drag of a standard projectile divided by the drag of the test projectile.
Either one of 'em means a great deal of nothing due to the number of variables involved.
http://www.hornady.com/ballistics-resource/external
"...it is a -LOT- of fun..." We all do silly stuff for exactly that reason.
 
Select your bullet from the drop down:
http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

Plenty of pistol bullets in there.

On edit, missed this:

In some looking here and there, it seems to me (unless I am mistaken?) that the BC assigned to the bullet is pretty much assigned by the bullet manufacturer and even then, it's kind of their opinion and it could be argued.
Click to expand...

What are you basing this off of? Pretty easy to calculate BC, all you really need is velocity data at the muzzle and various points down range. You really think when Sierra says their 9mm .355" 115 JHP has a BC of

.107 @ 1200 fps and above
.160 between 1200 and 1000 fps
.124 between 1000 and 800 fps
.140 @ 800 fps and below

...that they are just pulling that number out of their butts?
 
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When I want to estimate the BC of a bullet I go to the Lee Reloading page that lists their bullet molds. They list the BC's of each bullet they offer. I try to find one that is similar to what I am loading/shooting. Not an exact science but close.

I also scour the websites of the bullet manufacturer and I have been known to email and ask.

Hope this helps.
 
BC is based on a reference model. The old G1 model was for a one pound Krupp field gun projectile....

Instead of worrying about how a bullet that doesn't have anything close to the same shape as the Krupp reference projectile, just map out your actual ballistic data at ranges that you actually shoot.

After all, BC was created just for estimating the ballistic path for other projectiles so that useful ballistic tables could be calculated without so much real world testing.
 
Sghart3578,

That's a good method. Manufacturers used to work from tables produced in the 1930's that showed bullet nose forms, though these were all round and pointed noses, and didn't cover the flat meplat shapes you see today.

More recently, there are computer programs that will predict BC's. The most complete attempt at an online BC estimator I've run into is this one.

If you have a chronograph, you can use measured velocities to derive a BC, too, at least, for a certain velocity range. Just shoot a dozen over your chronograph at, say 15 feet, then back it up to fifty feet or so and do it again (assuming you trust your accuracy with the load enough not to shoot the chronograph). Just be sure the lighting conditions don't change between sets. Use the average (mean) velocities you get at each range and the difference between the first and second chronograph distance and plug the values into this calculator at the JBM site.


Emcon5,

While it is true that Sierra measures, not all the makers do. Many of them estimate and those that provide just one number generally pick it to apply only near the higher end of the velocity range they expect it to be used with (near common muzzle velocities for it). That's because the number tends to be higher at higher velocities for most rifle bullet shapes and the makers know they are going to be competing in a marketplace where the thing that tips the consumer's choice of which bullet to buy can be a higher ballistic coefficient, so they want the number as high as possible. Even with Sierra, when Bryan Litz measured their bullet's BC's he found their published numbers to be a little optimistic (though not enough to significantly mess up a ballistics table). I verified Litz's numbers for the .308" 168 grain MatchKing were right by running them against the drag function for that bullet that the U.S. Army Ballistics Research Lab had measured for M852 ammo originally. Their drag functions gave Litz's BC results.

Also interesting to note is that Litz has measured BC's varying 3% within a box of bullets. A box that averages a G1 BC of 0.475 might actually contain bullets with BC's anywhere from 0.468 to 0.482. That's probably due to the output of different sets of tooling being combined into the same box of bullets. The bottom line is there is always a limit to how precise this all is in practice.
 
To Uncle Nick,

Thanks for the info, that is a great suggestion.

I do use a chronograph occasionally. I will have to clear a range day and put it to use.
 
While it is true that Sierra measures, not all the makers do. Many of them estimate and those that provide just one number generally pick it to apply only near the higher end of the velocity range they expect it to be used with (near common muzzle velocities for it). That's because the number tends to be higher at higher velocities for most rifle bullet shapes and the makers know they are going to be competing in a marketplace where the thing that tips the consumer's choice of which bullet to buy can be a higher ballistic coefficient, so they want the number as high as possible. Even with Sierra, when Bryan Litz measured their bullet's BC's he found their published numbers to be a little optimistic (though not enough to significantly mess up a ballistics table). I verified Litz's numbers for the .308" 168 grain MatchKing were right by running them against the drag function for that bullet that the U.S. Army Ballistics Research Lab had measured for M852 ammo originally. Their drag functions gave Litz's BC results.

Also interesting to note is that Litz has measured BC's varying 3% within a box of bullets. A box that averages a G1 BC of 0.475 might actually contain bullets with BC's anywhere from 0.468 to 0.482. That's probably due to the output of different sets of tooling being combined into the same box of bullets. The bottom line is there is always a limit to how precise this all is in practice.
Click to expand...

Right, but we also have to look at the application the OP is talking about. This isn't a custom return to battery rig with 1/8 MOA sights, it is a handgun and he is talking about holding over the target.

Running the numbers in JBM for the Sierra bullet above, but manually entering the BCs of .160 and .124 instead of picking the bullet from the list, with a MV of 1300fps, the difference in drop is a whopping 3 inches at 200 yards and 10 inches at 300 yards.

The ranges he is talking about, with the equipment he is using, an approximate BC supplied by the manufacturer is perfectly adequate for his needs.
 
Right, but I'm only pointing out that Sevens is not off the mark to think estimating is commonly used in the industry, and so is publishing optimistic numbers for competitive reasons. That's separate from whether or not it matters much in his case. As your calculation shows, an estimate will likely be quite satisfactory for what he is doing.
 
Thanks guys. Yes, the bottom line is that I sit at a bench with an iron sighted revolver, barrel length of 4.2 to 7.5 inches and I send fat, stubby bullets at a leisurely pace anywhere from 100 to 375 yards... across a comical arc in some cases, with the goal of hearing a nice solid "CLANG" after what seems like an eternity.

Using the little app on my phone HELPS because it gives me a very good idea where to hope/expect my hold over will be, and it uses BC as part of that equation.

Particularly funny is when my buddy is spotting for me and he yells out "GOT IT!" but his statement is interrupted by the mild sound of the "CLANG" because he sees it and yells it before the sound gets all the way back to us. Ya gotta laugh, it's a lot of fun.
 
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