ok..well it consists of a psi chamber...it would have a end cap w/ a bike valve on the back...attached a bike pump..on top a psi gauge..on the opposite end one of those air compresser things...i dont know what there called but i have one...the other end of it would be epoxied in the barrel end cap...all you would have to do is pump the bike pump to the right psi indicated by the gauge and then squeeze the lever to release the air into the barrel to launch the tennis ball/other projectiles...
heres a rough sketch of what it would look like..









