What about refilling scuba tanks though? Aren't they also filled to 1000s psi?

Ok thanks. What about HPA though? How available is it, and what would prices be?

What would you guys recomend for using to refill a 3" 2' chamber to about 40-50psi in the middle of nowhere. Would a small portable aircompressor or bike pump be good? What about CO2 tanks?
Thanks.

Then where would I get a 3" tee that is pressure rated? Link to the acctual tee would be nice. I found one on mcmaster and grainger, both are sch 40, but mcmaster says it is rated at 0psi...

Ok, cool, I might try that. Homedepot or lowes wouldn't stock sch40 3" tees and fittings and such would they?

I have a friend with a metal lathe and is a highly skilled metal worker. Would an aluminum piston be a good idea?

So I checked out the sizes and such for fittings, and my original idea will not work. Would a 3" diameter polyethylene rod be able to withstand 100psi? http://www.mcmaster.com/#8701k55/=kt1yz
I am thinking about putting this inside a 3" coupler, I'll machine in o-rings and a check valve and such.

Also, does anyone know a good place to get 2.5" PVC and fittings?

I can probably get acess to the lathe at my school, the design I hand dress has a check valve and O-rings. It consists of 2 2" couplers and 2"-1/2" bushings inside them for the valve and bolts for the neoprene washers. My question is, will a 2" PVC coupler fit well inside a 2.5" tee?

Ok, so I have changed my idea again, because certain parts just dont fit together. I am thinking of cutting some polyethelyne rod up and putting a check valve in it, and bolting 2 segments together as a pison. Would this work? Would it withstand 100psi? http://www.mcmaster.com/#8701k55/=kt1yz

Ok, thanks to everyone who has responded. What about a 2 inch piston? I think I finally understand the concept pretty well. It would be nice though, if some people could post some pictures of them. I tried to look around to find some specifics, but wasn't too sucessful.

Would it really be worth getting a 2 inch sprinkler valve? What kind of ranges could i expect with a 3ft 2" barrel, 40-50psi, from either a 3" or 4" 2' air tank?

This is the first actual spud cannon/tennis ball cannon I have built, but I have mechanical experience. My question is, disregarding convience and such, which valve would be better? I have yet to find a 2" sprinkler valve, maybe I'm not looking in the right place, but I'd like to keep my valve price...