I have seen many threads asking about how to drive guns for a Mech using a microcontroller. I designed this circuit using TIP122's for a dual gun motor driver. I tried to stick with parts that are available from most Radio Shacks for people that don't want to order components online.
The driver also has a spot for an optional 5v power for the LED's to indicate when the driver is firing. This is mainly for troubleshooting and isn't required for normal operation.
The driver is limited only by the TIP122 so the continuous current should be 5A and it can peak to 8A. A heatsink would definitely be needed for the higher currents.