Step 11: Modify a servo
I chose to use a servo because they are relatively cheap and include a gearbox which gives them a lot of torque and limits their top speed. You'll have to modify a servo to use it as a drive motor. The Instructible below gives step by step instructions for modifying a servo.
TL;DR: rip out all the electronics besides the motor, attach leads directly to the motor, cut off the stop tab on the big gear.
I blew a couple servos when working on the gun, mainly by testing to see how much power they could handle. 18v is the danger zone, 25v is a burnt up servo. 12v works pretty reliably and spins the barrel at a decent speed.