What does Ballistic Coefficient mean?

[Marshall 357]

How about Sectional Density ? New to reloading just keeps getting more interesting.

 

[okie john]

Sectional density (SD) is a numerical expression of a bullet's weight in relation to its diameter. It doesn't really correlate to anything in real life. Rifle bullets with SDs over about .250 tend to penetrate well. Pistol bullets, on the other hand, have relatively low SD's, but tend to outpenetrate rifle bullets. Go figure.

Ballistic coefficient (BC) is a numerical expression of a combination of several aerodynamic properties. It's based on SD modified by several form factors, many of which reside mainly in the eye of the beholder. Bullets with a high BC fly flatter and drift less in wind than those with a low BC, but the difference has to be pretty large to matter. BC changes with velocity and altitude, and can differ significantly below or above the speed of sound, so be careful about hanging your hat on it. Bullet makers can't fudge SD, but they can (and do) get fairly creative with BC. High BC figures sell bullets, so the best way to determine BC is to ignore what the manuals say and shoot your own drop tests.

Picking a bullet solely for SD or BC is an error, as both matter far less than accuracy or terminal performance as based on their performance in actual tests in your guns. A knowledgable shooter will understand the concepts of SD and BC but avoid getting wrapped around the axle on either one.

Hatcher's Notebook has a pretty detailed description of both terms, if I remember correctly.


Okie John

 

[duckloads]

Thanks okie for the information. I've kind of wandered myself.

 

[Marshall 357]

Interesting Thanks

 

[jrm53]

I always wondered about the 2 terms but had no one to ask. Thanks

 

[DonD]

I'd say Okiejohn hit most of it.

I would say, however, that SD does affect BC. You could have a very streamlined slug whose form would be very aerodynamic with an extremely low SD and it would have a very poor BC.

Picture a slick Sierra Matchking BTHP shape that was mostly hollow, it would be streamlined but due to it's very light weight would lose velocity at an incredible rate and be useless for shooting.

I've never heard an adequate explanation of why big bore revolver slugs such as .475 Linebaugh and such firing heavy slugs at moderate velocities can frequently outpenetrate rifles with considerably more energy. Don

 

[DonD]

A little amplification here. I'm certainly not suggesting a hollowed slug is a practical example, it's exaggeration to illustrate a point. Perhaps my post was confusing to some.

If you had two slugs of identical shape and caliber and one had a lead core, the other a less dense material, the lead cored slug would have a higher BC and retain velocity better and penetrate deeper assuming the jacket held up adequately. Don

 

[model14]

I believe the .50 BMG Ball bullet has a BC of 1.0, and all other bullet BC's are determined by comparing their flight to it. Seems to me I remember reading that somewhere.

 

[DonD]

Originally posted by model14:
I believe the .50 BMG Ball bullet has a BC of 1.0, and all other bullet BC's are determined by comparing their flight to it. Seems to me I remember reading that somewhere.

I think the ball rounds BC is much lower than that. Hornady's 750gr AMax, an optimized target slug, has I think, a BC of 1.050.

If you're a really serious target shooter or sniper (not me), you need to check the trajectory on the range as Okie john said. The computer programs get you in the ballpark but not enough for really precise work. Don

 

[okie john]

Originally posted by DonD:
Picture a slick Sierra Matchking BTHP shape that was mostly hollow, it would be streamlined but due to it's very light weight would lose velocity at an incredible rate and be useless for shooting.

Exactly--like comparing a golf ball and a ping-pong ball. Comparing BC and SD assumes that the bullets are made of the same material.


Okie John