I wanted a Lego gun with a high rate of fire. Miniguns are easy to construct and offer extremely high rates of fire. It was the perfect choice here.
My minigun was a 4-barrel version. An 8-barrel version would have a higher ROF, but I didn't have enough parts. The barrels are connected to a central axle by two 40-tooth gears. The central axle is supported by the handle at the back and a beam at the front. A PF XL-Motor provides the power to rotate the barrels and launch the rubber bands. The battery pack serves as the handle and also supports the central axle. Everything is mounted on a rectangular platform, which also serves as the second handle of the gun (for my left hand).
The string I used was a 3m long Lego string I salvaged from one of my dismantled Technic models. Any other string would hypothetically work, as long as they can fit in the gaps between the pegs.
The rate of fire averaged at 11.2 rubber bands per second, which translates to 672 rounds per min.
Watch the video here: http://www.youtube.com/watch?v=eSYvSSlly54