Instructables

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.

http://www.instructables.com/id/How-to-modify-a-servo-motor-for-continuous-rotatio/step3/How-to-modify-a-servo-motor-for-continuous-rotatio/

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.