Gauss Rifles (as a 'coil gun') are some things I've built a few times and messed around with.

The problem they have, though, is control. You need very high speed, high power, high-voltage switching. That's expensive.

They can be fairly efficient if you can match the expansion and collapse of the field with the motion of the projectile. You have to watch 'ringing' in the coil - inductors are power storage devices, and that can cause problems when you don't want any power to be in the coil.

The other big problem is back-EMF creating a voltage spike on par with a tesla coil. That really makes use of semiconductors tricky.

I actually do not expect much from coil guns until the inherent problem with an inductor can be resolved. The inductor resists changes in the state of its magnetic field - and since you need the magnetic field to go from 'zero' to 'max' (and back to 'zero') as quickly as possible, this makes effective use a very difficult problem.

Room temperature superconductors is where it's at, with this one. Your actual coil may or may not be of superconducting material (probably best if it is not), but rather than using capacitors to store your power, you would use a superconducting inductor that you would drain into your firing coil(s). Since the current through the superconductor is nearly infinite (and an inductor converts its magnetic field into voltage necessary to maintain the current in the system) - you get almost instantaneous response from your firing coil, and the voltage with which you supply your system is irrelevant.

Then all of it will come down to switching control and preventing arcing.

A 'rail gun' on the other hand has its own problems. Notably is the corrosion of the rails. That will not be able to be solved until a physical connection with the rails is no longer necessary.

Both designs would function better in a vacuum - as has already been mentioned, the various atmospheric forces that come into play when you are sending a projectile down a barrel at mach 4+ can easily destroy your toy.

well still theres the act of reloading witc is a hastle and could get you killed on a battle feild so i would go for a rechargeing act of reloading so if your desprite you can over heat it and throw KABOOM and ether youve won or your out a gun and your screwd