What sort of range are you going for? Ours (range of ~70 yards) has a variety of components that make it work.
To start, we have a
compressor that lets up go up to as much as 120 psi in our
main tank. This goes through a standard FRC regulator to our holding tank (60psi max) made out of PVC. One end of the PVC goes into a
sprinkler valve that realeases all the air in our holding tank into the barrel. If you are interested in doing something like this, PM me and I can give you the details on how we control ours.