July 15, 2015 Posted by RedNex in Ammo, Blog, News

Gun Stuff 101 - Ballistic Coefficient

Ballistic Coefficient - by Josh

http://www.aerospaceweb.org

http://www.aerospaceweb.org

“In ballistics, the ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the negative acceleration — a high number indicates a low negative acceleration. This is roughly the same as saying that the projectile in question possesses low drag, although some meaning is lost in the generalization. BC is a function of mass, diameter, and drag coefficient.” (https://en.wikipedia.org/wiki/Ballistic_coefficient). In case you'd like to do the calculation yourself ... here's the formula for ammunition.

BCFORMULA

Sounds pretty complicated, but BC, simply put, is a measure of how a bullet flies through the air. Is it important to understand? To the person shooting short range rifle and pistol it is meaningless, but if you plan on spending some time at the bench shooting 100 yards and beyond it might help to have some understanding of BC.

Every bullet when fired has air resistance working against it. The design or construction of the bullet is critical in how well it overcomes this air resistance. Here’s an excerpt from Chuck Hawks web site that helps to clarify. “BC is what determines trajectory and wind drift, other factors (velocity among them) being equal. BC changes with the shape of the bullet and the speed at which the bullet is traveling, while sectional density does not. Spitzer, which means pointed, is a more efficient shape than a round nose or a flat point. At the other end of the bullet, a boat tail (or tapered heel) reduces drag compared to a flat base. Both increase the BC of a bullet. For example, a Hornady 100 grain round nose 6mm bullet has a BC of .216; a Hornady 100 grain spire point 6mm bullet has a BC of .357, and a Hornady 100 grain boat tail spire point 6mm bullet has a BC of .400. All three of these bullets have a sectional density (which is the ratio of a bullet's diameter to its weight) of .242, because they are all .243" in diameter and weigh 100 grains. But the more streamlined bullets have a higher ballistic coefficient. They are the ones to choose for long range shooting where a flatter trajectory is important.” (http://www.chuckhawks.com/bc.htm).

L to R: Round Nose, Spire Point, Spire Point Boattail

L to R: Round Nose, Spire Point, Spire Point Boattail

The same caliber bullets that weigh the same and fired at the same velocity will have different results just because of the BC. So, velocity is not nearly as important a factor as BC. Some of the load data on Hornady’s web site illustrates this better than I can.

www.hornady.com Left: 204 Ruger 24 Gr Right: 204 Ruger 40 Gr

www.hornady.com
Left: 204 Ruger 24 Gr
Right: 204 Ruger 40 Gr

The 24 grain NTX bullet is screaming out of the barrel at 4400 fps! (That’s fast) The 40 grain V max bullet has a muzzle velocity of 3900 fps (still pretty fast). Both rounds started with a 200 yard zero but look at the trajectory at 500 yards. The 40 grain bullet has five inches less drop than the 24 grain bullet. Now look at the velocity difference. The 24 grain bullet started out 500 fps faster, but at 500 yards we see that the 40 grain bullet is 500 fps faster. I checked Hornady’s data on these two projectiles and found that the 24 grain bullet has a BC of .170 while the 40 grain has a BC of .275.

So if you’re planning on some long range shooting, knowing the ballistic coefficient of your bullets can make all the difference.