The heavier the better. More energy. At the ranges you'd be using such a weapon there really isn't a need for anything lighter. It is possible to use helium as propellant rather than compressed air. Helium delivers more energy relative... something like near 50% more actually. They're designed to be charged to 3000 PSI. Some get better strings out of them charing on up to 3400 or 3800 psi. Regular old SCUBA tanks don't hold enough air for one, must have 4500 PSI 88 CF SCBA.