[I'mwithstupid^]

I think that it would be easy to do a pneumatic system that would be fast. You just use a double solenoid and a hopper (like a paint ball gun) and the ball falls down into chamber and than is shoved out by a plate on the end of a pnumatic system, than automatically the cylinder pulls back and another falls in and repeats, and if this isnt fast enough for you, these aren't complex shooting machines which means you could build two of them fairly easily.
you can have alternating firing mechanisms, so that one fires while the other reloads. Also you could use a pulley system to shorten the time of retraction for your springs. If you've got any corrections plz feel free