my advice: don't bother, you won't get a projectile up to any decent speed unless you have some beefy electromagnets and power source, not to mention firing systems and a decent projectile.
if you do decide to go with it however, it's gona be pretty big, and it depends on what type of propulsion your going for. if your just using magnets placed next to the barrel, it'll be pretty straight forward, but you'll have some problems with getting them to fire in order, however, if you plan on spiral magnets, good luck to ya
