I'd recommend using a compressed air cannon. Much more predictable, and much less sharp pressure spikes. Add in a foot-pedal bike pump, and you can get preciously close to the 100 psi sanity cutoff point. And you can get a ton more oomph, too.
My gun, the Dragonlance Mk.1 prototype (a former classmate has the first Mk.1 Production) is almost stupidly simple to build. Get five feet of 2" PVC, a 2" cleanout cap, and a 2" solvent weld ball valve. You'll need some PVC cement, and a Schrader valve from the auto parts store for filling. Cut the PVC into two pieces, one about 2 feet long to be the pressure chamber, and one about 3 feet long to be the barrel. Put the cleanest cut on the muzzle end of the barrel for looks. You'll want to drill a biggish hole in the short section of the pipe, about three inches from the end to be the stock, but use the smallest one you can get away with and still have the valve stem seal. Now comes the welding. Use lots of the purple primer, and don't be chinsey on cement. Remember, this is going to have to hold in a decent amount of energy. Take the center out of the cleanout plug, and cement it to the valve stem end of the chamber (or the other end; this is personal preference - I like mine out of the way). Then cement this to the ball valve, making sure the two line up; this'll help keep weight off of the valve if the gun is set on its side and rolls. Finally, cement in the barrel and let it sit for... oh, 24 hours is good, but 72 is better. Then, finally, check the valve for function (find one in the store that's a bit loose, most are really hard to open) and screw on the endcap with a bunch of teflon tape.
then air it up to increasing pressures, in 5 or 10 PSI increments, and launch convenient projectiles to get a feel for what it's capable of. My original uses Sch40 PVC, and I've used it at 100 psi a few times. Keep it out of the sun, and paint it -- I recommend a flamed barrel end, though this never got past some Sharpie on the prototype.
Another thing. Dixie cups are just the right size to make self-sealing sabots.
My gun, the Dragonlance Mk.1 prototype (a former classmate has the first Mk.1 Production) is almost stupidly simple to build. Get five feet of 2" PVC, a 2" cleanout cap, and a 2" solvent weld ball valve. You'll need some PVC cement, and a Schrader valve from the auto parts store for filling. Cut the PVC into two pieces, one about 2 feet long to be the pressure chamber, and one about 3 feet long to be the barrel. Put the cleanest cut on the muzzle end of the barrel for looks. You'll want to drill a biggish hole in the short section of the pipe, about three inches from the end to be the stock, but use the smallest one you can get away with and still have the valve stem seal. Now comes the welding. Use lots of the purple primer, and don't be chinsey on cement. Remember, this is going to have to hold in a decent amount of energy. Take the center out of the cleanout plug, and cement it to the valve stem end of the chamber (or the other end; this is personal preference - I like mine out of the way). Then cement this to the ball valve, making sure the two line up; this'll help keep weight off of the valve if the gun is set on its side and rolls. Finally, cement in the barrel and let it sit for... oh, 24 hours is good, but 72 is better. Then, finally, check the valve for function (find one in the store that's a bit loose, most are really hard to open) and screw on the endcap with a bunch of teflon tape.
then air it up to increasing pressures, in 5 or 10 PSI increments, and launch convenient projectiles to get a feel for what it's capable of. My original uses Sch40 PVC, and I've used it at 100 psi a few times. Keep it out of the sun, and paint it -- I recommend a flamed barrel end, though this never got past some Sharpie on the prototype.
Another thing. Dixie cups are just the right size to make self-sealing sabots.