They'll hold about 100 psi, but they CAN burst at lower pressures. Don't build it in .22 cal, .22 only has.037994 square inches of area, so the gas would only have that little space to act on. That would give AT MOST 3.8 fpe at the muzzle.
However building it in .50 cal would give you up to 20 fpe(kinda....dont feel like doing much math thats the gist of it).
Pellet guns can get such high velocities in small calibers b/c theyuse very high pressures, butuse avery low volume of air.
High pressures are dangerous to safely contain(don't quote me on the 10,000 psi gun!!!!You're a noob to this I have experience building airguns!!!) but lower pressures with a greater volume of air can providereasonable power.Foryotes use a 1/2 inch pvc barrel witharrow shafts and mechanical broadheads. If you can get a gun to shoot a marble through 1/2" plywood,you should be good for ayote head shot(at reasonably short range).

Try www.spudfiles.com again, people are helpful there. Start with a sprinkler gun, dont go for a piston-valve when you first start.