Why Two Ballistic Coefficients for Rifle Bullets?

If you pay attention to the information on bullet boxes, you've probably noticed that some of them give you two ballistic coefficients (BCs) for the projectiles therein. For example, a box of 143-grain 6.5mm Hornady ELD-X lists a G1 BC of .625 and a G7 BC of .315. Hornady is not trying to make your life more difficult; it's merely trying to be a little more informative about what it's selling. Here's the story.

The science of exterior ballistics goes all the way back to the Renaissance—Galileo was interested in it, as, later, was Sir Isaac Newton. Most interested of all were artillerists, since their ability to rain death and destruction accurately was determined in large part on being able to predict a shell’s flight through the air.

By the time of the Civil War, there were artillery pieces that were capable of firing a projectile 5 miles, the most prominent being the British Whitworth Rifled Cannon. But there were problems, and one of the most common was figuring out just how far a shell would go when you set the muzzle at a particular elevation.

By the time Europe was cranking up for The War to Make World War II Possible, gunnery had progressed by leaps and bounds. Cannon-cockers of all nations had now mastered the science of indirect fire (shelling something that was beyond your range of vision), and one of the tools they used for this was the ballistic coefficient, which is a mathematical expression of a projectile’s ability to slice through the air. If you knew the BC of your shells, you could predict where they would fall.

The first BC, the long-familiar figure we’ve seen on bullet boxes since Harry Truman was an artillery captain, is G1. This formula is used to determine the coefficient of square-based, blunt-nosed projectiles, which is what they used in World War I artillery pieces. However, it’s not ideal for calculating other shapes. Thus, over time, we’ve evolved G2, G5, G6, G7, G8, and GL.

Makers of streamlined bullets have picked up on G7 because it’s designed for projectiles with a long, 7.5-degree boattail, and a 10-calibers tangent ogive, which is a pretty good description of a highly streamlined rifle bullet. The problem is one of familiarity. We’re so used to thinking in terms of the G1 BC that, when confronted with G7, we snort and fart and wonder why this super-streamlined slug has a BC of only .300 and change. The G7 figure is far more accurate, but who can relate to it? Not I. It’s like when a speaker of British English says that someone weighs 16 “stone.” A “stone” is 14 pounds, so in my head I calculate that 16 times 14 is 224 pounds, or else I lose interest and walk away muttering.

So, sensitive and caring bullet makers list both G1 and G7. As for me, I continue to think in terms of G1, and refer to the icebox, and the phonograph, and think of people who ride on airplanes as passengers instead of customers, because that’s what I learned 70 years ago.