Railgun ammunition is designed to lose as little kinetic energy as possible. After all, there's no point hitting someone with a non-explosive projectile unless it's moving fast.
Re:Navy Sets World Record With Rail Gun
by pingswept on Friday December 10, 2010 @05:31PM
(#1993450)
This suggests that the terminal velocity of a rail gun projectile is around Mach 5: http://www.globalsecurity.org/military/systems/ship/systems/...
I guess I can spend my time worrying about other things!
I'd expect that most projectiles decay exponentially toward a terminal velocity, so the impact of shooting a bullet faster is negligible when it's going to be in flight for 6 minutes. Anyone know whether that's true for long range projectile weapons these days?