There's been plans on the net for a while for coil guns, but they're all one shot deals. Equivalent to a muzzle loader if you will. The basic idea behind one is to energize a coil to pull a projectile through the coil, and shut down the field when the projectile reaches almost half-way through and it should keep sailing on.
Factors like size of projectile, size of coil, voltage, amperage, starting position of projectile, number of coils, and so on all have a big impact on how well it works. I found some site where some guy did his homework and had it all mapped out. Nice and cheap coil gun, only fires at about 12 meters/second. Great for fun, not dangerous.
To build on the "fun" part of it, what if it could be made fully automatic? Have an optical sensor at the exit hole, and once the projectile passes through it, activate a stepper motor that loads the next dry wall screw, stick of metal, whatever into position? So long as you keep the hopper full, and maybe use a gear with deep teeth to grab the next projectile and stick it in the firing hole. Once the new projectile is in place and trips the next optical sensor, the coil energizes again and does its thing, while the gear is rotated back into "grab next round" position. Fully automated process until you run out of power or ammo!
How feasible would it be to rapidly create and collapse a magnetic field anyway? Isn't that a function of inductance?