The Ballistic Coefficient of Rifle Bullets
By Chuck Hawks
Ballistic Coefficient (BC) is basically a measure of how streamlined a bullet is; that is, how well it cuts through the air. Mathematically, it is the ratio of a bullet's sectional density to its coefficient of form. Ballistic Coefficient is essentially a measure of air drag. The higher the number the less drag, and the more efficiently the bullet cuts through the air. So for purposes of flying through the air efficiently, the bigger the BC number the better.
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.
To illustrate the practical difference between these three styles of bullets, let's use Hornady's trajectory figures for the 100 grain 6mm bullets above. Starting all three bullets at a muzzle velocity of 3100 fps from a scoped 6mm rifle zeroed at 300 yards, the trajectories are as follows.
.243" 100 grain Round Nose (BC .216): -1.5" @ muzzle, +4.8" @ 100 yards, +6" @ 200 yards, 0 at 300 yards, -15.9" @ 400 yards, -46" @ 500 yards.
.243" 100 grain Spire Point (BC .357): -1.5" @ muzzle, +3.8" @ 100 yards, +4.7" @ 200 yards, 0 @ 300 yards, -11.1" @ 400 yards, -30.5" @ 500 yards.
.243" 100 grain Spire Point BT (BC .400): -1.5" @ muzzle, +3.6" @ 100 yards, +4.4" @ 200 yards, 0 @ 300 yards, -10.4" @ 400 yards, -28.6" @ 500 yards.
There is a pretty big difference in trajectory between the round nose bullet and the two pointed bullets, making it obvious why it is folly to choose a round nose bullet for long range shooting with a high velocity rifle like a 6mm Remington or .243 Winchester. Also notice the big difference in BC between the round nose bullet (.216) and the spire point bullet (.357).
But there is less difference between the trajectory of the flat base spire point bullet and the boat tail spire point bullet. The boat tail helps, but not nearly as much as the point on the front of the bullet. The boat tail bullet had .3" inch less rise at 200 yards, and 1.9" less fall out at 500 yards. These differences are real, but unlikely to make or break a shot at a big game animal. This is shown by the smaller difference in BC between the two pointed bullets, .357 for the flat base and .400 for the boat tail.
To further assess the importance of a boat tail, note these pairs of Speer spitzer bullets of the same weight and caliber. In each pair, the first bullet has a flat base, and the second has a boat tail.
.243" (6mm) 100 Grain BT, BC .430
.257" (.25) 100 grain, BC .369
.277" (.270) 130 grain, BC .408
.308" (.30) 165 grain, BC .433
A list of pointed (spitzer type) hunting bullets considered a good bet for long range shooting in their respective calibers, with their ballistic coefficients, follows. Boat tail bullets are designated "BT," all other bullets have flat bases.
All of the figures that follow are taken from the Speer Reloading Manual Number 13. The same weight bullets from other manufacturers will have different BC's, because they are slightly different shapes. But these Speer numbers are as typical as any, and being from the same source they are useful for purposes of comparison.
.243" (6mm) 90 grain, BC .385
.243" (6mm) 100 grain BT, BC .430
.257" (.25) 100 grain BT, BC .393
.257" (.25) 120 grain BT, BC .435
.264" (6.5mm) 120 grain, BC .433
.264" (6.5mm) 140 grain, BC .496
.277" (.270) 130 grain BT, BC .449
.284" (7mm) 145 grain, BC .457
.308" (.30) 150 grain BT, BC .423
.308" (.30) 165 grain BT, BC .477
.311" (.303) 150 grain, BC .411
.323" (8mm) 150 grain, BC .369
.338" (.338) 200 grain, BC .448
.375" (.375) 270 grain BT, BC .429
Note that with bullets of the same weight and style, such as the 100 grain .243" and .257" bullets or the 150 grain .311" and .323" bullets, the smaller diameter bullet always has the superior BC due to its better sectional density.
As examples of very streamlined bullets, note the BC of these Speer match type bullets. These are all pointed hollow point, boat tail bullets.
.284" (7mm) 145 grain Match, BC .465
.308 (.30) 168 grain Match, BC .480
.308 (.30) 190 grain Match, BC .540
Note that in the .308 pair, the heavier bullet (which has the greatest sectional density) has the better BC. The extremely poor SD of the .224" bullet lowers its BC, even though its shape is similar to the others.
Which explains why .22 bullets drop so much at long range and are so subject to wind drift, compared to larger caliber bullets with superior sectional densities (and hence, BC's). At a MV of 3100 fps a .224" Speer 52 grain BTHP Match bullet zeroed for 300 yards has a 500 yard drop of -43.9 inches, not much better than the 6mm round nose bullet in our trajectory examples near the beginning of this article. An interesting subject, this ballistic coefficient, and worth paying attention to when you select a bullet (or a caliber) for long range shooting.
Copyright 2001, 2010 by Chuck Hawks. All rights reserved.