In WWII the air force (I can't remember whether British or American) needed bombs to destroy dams, so they decided on giant ball bombs spun to give backspin to keep them up next to the dam. Anyway they did tests and found that a dimpled ball flew further with more accuracy than a smooth ball.
I never even thought about a superconductor but that is about the only way the claims of the manufacturer can be true, but then you would not be able to use tungsten ammo and air resistance might melt the bullets.
I am studying physics in school, and I think that you are forgetting centripetal force, the reaction force happens before the projectile leaves the weapon. That force is the projectile pulling out away from the center of the centrifuge and when the projectile leaves the weapon the force goes...