Only one problem. It needs 6 brand new D batteries to get anywhere close to the claimed 3 shots/sec. Those aren't cheap, and need to be recycled.
Fortunaately, there's an easy fix. 6 x 1.5V batteries is not too far off from a 9.6V rechargeable racing pack.
Parts:
- 9.6V racing pack. You can get a decent one with a charger for $15. Try to get a charger which shuts off automatically when the battery's charged; the cheap ones don't, which will fry your battery if you forget and leave it plugged in too long.
- Extra connector for racing pack. I chopped mine off one of those cheap chargers, which came with a remote control missile launching tank.
- Alligator clips.
- Wire cutters
- Soldering iron
One end of the battery box has metal pads on it. If you hold the box so those are on the left, that matches my picture. Clip the - (black) wire from the battery pack onto the spring closest to you. Clip the + (red) wire from the battery pack down into the far right connector. It should look like this:

Disclamer: Use this mod at your own risk. I'm sure this voids the warranty. While I haven't seen the slight over-voltage burn out the motor of my Vulcan or break the internal action yet, that doesn't mean it won't happen eventually. This mod is not endorsed by Hasbro.