John you can have the one i have.

Now all we need to do is teach John-John how to shoot those recon helo's!!

Heres an interesting design I found on ebay. Should be able to do it for relatively cheap.

Paintball Cannon! bazooka grenade launcher ion tank a5 - (eBay item 320197419040 end time Dec-23-07 17:34:56 PST)

the eBay cannon is also a "dirty" full metal up against a valve then open thru the barrel. The problem here is that 1) you have to be quick and consistent on open the ball valve quickly by hand, and 2) this is easier, safer & more accurate if you add an inline reg.

and , YES, YES, AND YES...ANYTHING WITH A PVC CHAMBER MUST HAVE A 110 OR LESS RELIEF VALVE.

for a compact design this really means you're limited to straight through the barrel and/or use a metal chamber for the higher pressure (this is what the $400 MKS ones do)

FYI:axial designs can get shorter, but gain diameter and weight, however the high volume lends itself to ALOT OF ENERGY with very low pressures (ask DBD what 60 psi can do with my remote controlled axial cannon)

my .02