Again, where the main tank comes in and attaches isn't shown yet, it would be like Dr. Nerf's though. I was thinking have some vinyl tube come in the side of the PVC, it shouldn't need a great flow rate.
That's one problem. The one I was talking about is how it will refill. The intake tube is in the wrong place. The only solution I see is to move the air intake, or add another oring (I have a picture in my head, but I can't really explain it. If I were you, I'd move the handle more towards the front (where the intake is) and move the intake back. That would solve your problem.
The air intake from the main supply is in between the left and middle O-rings. When the middle O-ring is in the camber it's not blocking any air and the chamber fills.
I got the inspiration from Dr. Nerf's design. I figured bull-puping it would about cut the length in half. I like the bull-pup design because it reduces your length signifficantly.