Nice experiment! Now the OP can just focus on simplicity.

Many interesting and good things have been said. I think it's all been covered. I'd just like to suggest maybe trying X-shaped o-rings. I tried them and love them. They are less picky about being squeezed too much and having too much friction while still sealing perfectly. They are made for sliding ...

Any update on this?

........lol..........

I like the machine work you did. I love projects like this. I did the grease gun thing like mrfoo said, and it worked for me. I filled the cannon with water, then pumped grease in. I've read that you can modify the grease gun to pump water in, but all that did was leak for me. There's a way to do it...

I'll just stick with my bike pump.

I should offer the police my hybrid design with 3 minute rate of fire between shots

That is impressive. So compact yet powerful. Very well done!

I'm pretty clueless about what you need. Maybe some more details about the cannon might help. There are some pneumatically minded guys around here who will likely chime in. The only thing I really know how to do is make a piston cannon. Sounds like you want to buy a valve, but I can't think of any p...

A cup seal might work, or you could use an oversized X-shaped o-ring and have it compress more than normal. It might fill in the gaps. Have you tried a different piece of PVC pipe? Another option might be to use a hose clamp around the PVC pipe to force it to conform to the piston housing. If the pi...

I think the valve looks great. That looks like the kind of thing I would design myself and I like it. I haven't studied the valve closely, but it just looks good generally. I wonder how strong the plastic will be where those small screws are. The only real concern I have is you mentioned something g...

I'd buy a high-end PCP airgun and swap the barrel with a smooth one for BBs and just use that. You can connect the gun to a large air tank.

Watching a piece of 3" PVC hold until 1000psi+ makes me wonder what it would take to cause my aluminum cannon to burst. I'm starting to think 3.5X combustions just might not have the energy to cause that thing to come apart.

Am present. I made a new cannon in 2019 I think it was, and it was quite a project. The site is very low in activity, the glory days of spudding seems to have been in the 90's and early 2000's. Spudding will always have a special place in my heart and I have two special cannons that I will either le...

https://www.youtube.com/watch?v=KqstP9ics2A Great video. He tested hydro testing to failure with PVC and it has enough POP to hurt someone for sure. Not sure if there was air in the line behind the water, but an interesting result. Lots of cool PVC failure testing specifically with spud guns in mind.