From what I remember off the top of my head, it was an M-16 A2. (I'm sure someone with better memory will correct me.) Oddly, the upper receiver is flipped around (not including the carry handle). Forward of the receiver is an M203 grenade launcher shroud. That piece seems to be puttied into the receiver. I dunno if anyone's ever IDed the fuel bottle, the pistol grip or the ignition chamber. I've always presumed that they were custom fabricated parts. I expect someone here has better info. I haven't checked the Aliens Legacy board for info on it in a long time, but there might be something useful there. I've always wanted to build one myself, but it hasn't gotten to the top of the list yet.