Slugs

[GPConway]

At the risk of pretending to be ballisticboy, I must point out that BC changes with velocity.

I'm not pretending to be Ballisticboy either but if the Ballistic Coefficient is calculated to be changing at different velocities then you're calculating against the wrong/inappropriate Drag Curve ... or perhaps you're using a 'constant Cd' approximation.
Maybe you're thinking about the Drag Coefficient - which does indeed vary substantially with velocity.

George