Looking at the in-game descriptions, we're dealing with coilguns (e.g. a ferromagnetic projectile being pulled along a barrel with coils being turned on in sequence, or in Perpetuum's case, a single coil pushing the projectile away, given their descriptions). They are usually referred to as Gauss guns.

I'm just wondering, what's the difference between the EM-variety and the Gauss-variety? The range difference suggest the EM-variety might be a railgun, but that would contradict the descriptions for them.

Railguns generally involve two rails and a conductive slug (doesn't need to be ferromagnetic). A massive discharge of current on the rails, with the slug closing the circuit between the rails, will accelerate the slug along the rails according to the right-hand rule (magnetic fields, Lorentz force, and all that science). Because there's no magnetic saturation (which limits the speed a coilgun can accelerate a projectile to), the theoretical speed is extremely high (think 20+ km/s).

http://en.wikipedia.org/wiki/Coilgun
http://en.wikipedia.org/wiki/Railgun