Just how much influence does a bullet's shape have in supersonic flight? Obviously, the magnitude of drag has a lot to do with bullet shape, but what about the drag profile? Can the G1 ballistic coefficient be used to accurately predict the trajectory of a bullet in supersonic flight when that bullet looks like the G7 standard projectile?

Now that Berger Bullets is publishing both the G1 and G7 ballistic coefficients for their VLD bullets, it makes it easy to do some comparisons. Beger's #24530 has a diameter of 0.243 inches and a weight of 115 grains with a published G7 BC of 0.279 and a G1 BC of 0.545. That gives it a sectional density of 0.27822 lb/in� for a G7 form factor of 0.9972 and a G1 form factor of 0.5105. A form factor of 1.0 is the exact match for the referenced standard projectile, so Beger's #24530 is nearly a perfect form factor match to the G7 standard projectile and a poor match for the G1 standard projectile, and thus, any difference in drag profile would show up with this bullet.

Given a 3,500 f/s MV and using the equal time of flight to 1500 yards conversion of the G7 BC produces a G1 BC of 0.559, which is about 2.6 percent higher than the published G1 BC. You can see in the Drop values below that the G1 BC of 0.559 is a better match than the published G1 BC of 0.545 over the 1500 yard range. No doubt Beger is attempting to match their bullet's characteristics into the subsonic range, something I have the luxury of ignoring as I don't intend to go below Mach 1.2 for long range shooting.

  Rng Yards --> 100  300   500   700    900    1100    1300   1500
  G1 BC 0.545: -1.5 -14.4 -43.6 -93.7 -171.0 -284.0 -445.2 -671.2
  G7 BC 0.279: -1.5 -14.3 -43.0 -91.9 -167.0 -276.4 -431.5 -649.4
  G1 BC 0.559: -1.5 -14.4 -43.4 -92.9 -169.0 -279.8 -436.6 -655.2
Drop values are from JBM website using 0 altitude, 78% humidity, 29.92 in Hg corrected, 59 F, 0 sight height, all boxes at bottom unchecked.

The difference in drop between the G7 BC 0.279 and the G1 BC 0.559 at 1500 yards is 5.8 inches (0.4 MOA) and equivalent to a change in shot to shot muzzle velocity of just 14 f/s.

I've looked at a number of examples and the difference between G1 and G7 trajectory calculations is minimal to ranges where velocity gets down to around Mach 1.2. The difference between G1 and G7 shows up in the transonic velocity range, which it seems few long range shooters use. Apart from magnitude, apparently the drag characteristics of supersonic flight are insensitive to bullet shape. Thus, you can't look at the shape of a bullet and tell which standard projectile it best matches in supersonic flight as it doesn't seem to matter much what the shape is. Maybe that explains why the industry seems reluctant to follow Beger in publishing both the G7 and the G1 ballistic coefficients.

Credit for the conversion to different drag functions using equal time of flight to some distance belongs to Ken Oehler who sometimes visits the "Ask The Gunwriters" forum. He described the technique in a July, 2007 Shooting Times article. Ken's conclusion is that bullet manufactures should measure ballistic coefficients over long ranges similar to what the bullet is intended for. Seems like that would give us BCs that better match the intended purpose of all long range bullets.