Drag Model
Also: G1 · G7 · G7 BC
The reference projectile a bullet's BC is measured against. G1 is a flat-base reference; G7 is a boat-tail reference.
A G7 BC of 0.300 is roughly equivalent to a G1 BC of 0.585 for the same bullet. The numbers aren’t interchangeable because they describe drag against different reference shapes.
For long-range work past 800 yards, use G7 if the manufacturer publishes it. G1 numbers tend to be inflated because the reference shape drags more than a modern boat-tail bullet, making the bullet’s relative number look better than it is.