It’s very awkward to hold with no stock (that’s phase 2), but I did a couple of test shots.

Basically I’m going to use stainless steel bands to clamp a u channel to the bottom of the cannon. Onto this I’ll mount a butt, grip and foregrip. The butt will be milled hdpe.

I’ve been playing with this design in cad for a couple of days. A couple of thoughts:

I considered many methods of attaching the u channel to the cannon tube. In the past I’ve always used hose clamps, bur I wanted to go with something nicer this time. The U-channel doesn’t attach to the pressurized portion of the pipe so welding is an option, but I don’t know how nice it would look with my tig skills being what they are. It would also make working inside the u-channel a pain.

I’d originally planned on cnc milling the grip and forgrip out of hdpe in addition to the butt, but I think the gun parts will be easier and give it a cool prototype-weapon look.

Woah! Look at this:

I wish I had found this before. This guy Damon Williams designed his own version of the Metadyne launcher. And posted full plans!
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Very interesting and useful. I can’t quite figure out how the poppet valve on the trigger works in his design. ..It has to act like a 3-way valve somehow. This is incredibly cool.

I eventually settled on using fixed length bands to attach the pipe to the stock. The bands are bolted onto aluminum blocks (the red blocks in the picture). Those blocks are in turn bolted onto the The U-Channel that forms the frame of the stock. I thought this arrangement was pretty clever, since tightening the bolts also tensions the bands at the same time.

I started by drilling the holes in the u and making the blocks. All the pieces are square and easy to fixture so it went really fast.