Another option if you don't plan on carrying a full rig is a simple couple of straps i rigged up on my p90. I used 2 lengths of nylon webbing and some velcro to strap the mag to the bottom of the gun. as long as you put it on with the feeder towards the top of the gun mag swaps are easy. Here is a pic of it mounted on the gun:
http://i174.photobucket.com/albums/w107 ... MG0184.jpg
And everything separated:
http://i174.photobucket.com/albums/w107 ... MG0185.jpg