It's an AT4K integrated into the bottom of a crutch. Here are some more pictures.
A view down the barrel, which is 17/32 brass nested in CH40 PVC pipe.
The stock has two peices of CH80 CPVC for ammo holders shoved in the ends of the pipes.
Here's a size comparison with my BBB.
Edited by Firefox551, 27 September 2008 - 01:03 PM.