The Firing Line Forums - View Single Post - How is ballistic coefficient/coefficient of form determined?

Gopher a 45 · March 15, 2000, 12:56 PM

Sensop,

Thanks for the info. It's just as I suspected. The coefficient of form is a pain to derive! All the reloading manuals sort of give a: "Trust us. The BC is what we say it is" without any elaboration as to how they get an actual figure of say, .269 for a 55g .223FMJ bullet. They're happy to provide the formula for BC, but don't provide the way to get one of the variables, coeff. of form. Undefined variables make me nervous! So I was wondering how they got their numbers. I suppose Speer et al. have got a lot of eggheads working with slide rules and abaci back in the shop somewhere to come up with this stuff! I'll have to look into getting those volumes you mentioned. I assume they're still available? I see now why a 40mm Bofors round at 3200fps has a better BC than a .243 round of the same shape and velocity. It's all about mass! I guess our old friend Newton's at work again. The inertia of the larger round resists velocity loss due to friction much better than the varmint bullet. I guess the corrolary is you've got to use more bang-stuff to get it moving at the same velocity and therfore recoil is greater. Man, there just ain't no such thing as a free lunch when it comes to physics! As an aside, I know the Jerries were into big guns. Didn't Krupp make all the German battleship and railroad guns including that monster Paris gun of WWI? Talk about an exercise in ballistics! I think that's where they discovered the Coriolis effect, since Paris wasn't where they were aiming at by the time the shell had traveled the 75 miles to the target. You know you've got a long-range gun when you have to factor in not only windage but the rotation of the earth! I wonder if anyone makes reloading dies for something like that

So, I guess it would be about time to do some new testing if the only tables out there are based on work done 75+ years ago? Of course, physical laws haven't changed since then (as much as Congress tries), so maybe it would be a waste of time.

Ricciardelli,

Sure, I get those conditions here at my range all the time. You mean you don't? And, rocket-assisted projos give a consistent velocity every time (my gyrojet pistol functions flawlessly).

March 15, 2000, 12:56 PM	#4
Gopher a 45 Senior Member Join Date: January 24, 2000 Posts: 329	Sensop, Thanks for the info. It's just as I suspected. The coefficient of form is a pain to derive! All the reloading manuals sort of give a: "Trust us. The BC is what we say it is" without any elaboration as to how they get an actual figure of say, .269 for a 55g .223FMJ bullet. They're happy to provide the formula for BC, but don't provide the way to get one of the variables, coeff. of form. Undefined variables make me nervous! So I was wondering how they got their numbers. I suppose Speer et al. have got a lot of eggheads working with slide rules and abaci back in the shop somewhere to come up with this stuff! I'll have to look into getting those volumes you mentioned. I assume they're still available? I see now why a 40mm Bofors round at 3200fps has a better BC than a .243 round of the same shape and velocity. It's all about mass! I guess our old friend Newton's at work again. The inertia of the larger round resists velocity loss due to friction much better than the varmint bullet. I guess the corrolary is you've got to use more bang-stuff to get it moving at the same velocity and therfore recoil is greater. Man, there just ain't no such thing as a free lunch when it comes to physics! As an aside, I know the Jerries were into big guns. Didn't Krupp make all the German battleship and railroad guns including that monster Paris gun of WWI? Talk about an exercise in ballistics! I think that's where they discovered the Coriolis effect, since Paris wasn't where they were aiming at by the time the shell had traveled the 75 miles to the target. You know you've got a long-range gun when you have to factor in not only windage but the rotation of the earth! I wonder if anyone makes reloading dies for something like that So, I guess it would be about time to do some new testing if the only tables out there are based on work done 75+ years ago? Of course, physical laws haven't changed since then (as much as Congress tries), so maybe it would be a waste of time. Ricciardelli, Sure, I get those conditions here at my range all the time. You mean you don't? And, rocket-assisted projos give a consistent velocity every time (my gyrojet pistol functions flawlessly).