Satisfied with the forearm, I decided to give the gun a shot (pun intended). Near the end of the barrel there's a round bulge. I decided to use multiple blocks of styrofoam with a circular core cut out.
After getting a few of them ready, I slipped them over the ABS tube that is going to act as the gun barrel. After a little bit of glue between each segement, I pushed them together, let them dry, did a bit of carving, and ...
... ended up with something reminiscent of styrofoam halal meat. Satisfied this stupid bit of mickey mouse engineering was going to work, I carved a rough bulge and set it aside to be sanded and shaped later.
The last bit that I worked on was the butt of the gun. It's supposed to be a half octagon shape but there's no way I'd be able to hold it with the bottom smashing against the forearm. So... it got neutered a little bit. The idea is to sandwich 3 pieces of foam with a a rectangle cut out. The ABS tubing will slide into the hole and two more pieces of styrofoam cut to shape will be glued to the top and bottom.