That Guy on page 372
@Personface A gauss gun uses sequential powerful electromagnets to pull a ferric projectile down the barrel. Usually these are coils, hence the nickname of "Coil Gun". A rail gun uses two parallel conductive "rails" with the projectile seated between touching both rails. By sending a charge through the circuit, the projectile is accelerated. It's functionally very similar to a homopolar motor, but linear rather than rotational. Usually it is easier to build a rail gun, but you can get higher muzzle velocities from a gauss gun.

Maple on page 372
It's also possible that that isn't a gun at all, and that he wanted to see how Charlie would react. Now, chrono-assassin knows that Charlie will try to kill him if given the chance.