I'm brand new to all of this - I saw the video on how to make a piston valve and decided I needed to try it. And it was tons of fun. That said, I'm totally lost looking at your diagram. I need to go do some reading about detent and burst disks. Looks like another fun project. Thanks!

Thanks - I guess this just isn't going to be shooting tennis balls if the valve is the main problem.

Like I said in my original post, GGDT said my barrel was too long, but I thought maybe that had something to do with my guesstimates.

How did you calc that chamber:barrel ratio?

I made this exact piston valve (MrRedRooster) http://www.youtube.com/watch?v=2bvXiswEDSo I put an adapter on there so it has a removable barrel, and removable canister. I made a 3/4" marble/acorn barrel that works beautifully with a 1L metal cylinder. Then I made a 2"x36" PVC canister for a 2.5" x 6...

First of all, there is nothing that isn't totally awesome about that photo. I read your post about that cannon - very impressive. I built one of the piston QEVs i found on here (finally found a solution to the hose barb needed - I used a pipe nipple instead) The smallest opening in there is about 1/...

Sorry, not enough detail - I want to use these for air canisters. I'm having trouble finding a fire extinguisher bottle, so I thought this would be the next best option. But I want to know the down side to two vs one canister. Due to the way they are built, they are not able to provide high flow rat...

I have a couple used propane canisters like the one in the pic below, neither of which is big enough for what I want to do, but both combined would work well. Can I just connect the two with a Tee? What are the cons to a set up like this? http://www.pastrypal.com/wp-content/uploads/2009/10/puff-past...

I'm hoping to put a potato into low earth orbit, but i'll settle for a few football field lengths.

Thanks for the 2x orifice rule of thumb. That's exactly what I was looking for.

Is there a similar rule of thumb for canister size and barrel size/length?

I am building a cannon with an interchangeable barrel so I can shoot anything from marbles (~1/2" barrel) to full potatoes (~2" barrel).

If I use the ebay QEVs mentioned in this thread, will that work for a 2" barrel? Is a 1/2" valve too small?

Thanks!

I'm in the States - no Burnnings around. Thanks though. I found a hose barb in the irrigation section at Home Depot, but as others have said, the threads were much coarser and will not work. It would be a lot more fun to make my own QEV, but I think this ebay item is my only option for now. Thanks f...

So can you use that ebay QEV to replace this brass valve in the how to section? http://www.spudfiles.com/forums/self-closing-piston-valve-brass-fittings-t17609.html I'm trying to build the gun in the video below, but I can't find the hose barb anywhere around here (or on the internet for that matter...