SpudFiles

A QEV has higher flow and opening time, giving you more power for the sacrifice of less shots. It also means the hammer doesn't need to be as strong as it's fighting a smaller surface area, allowing larger bores.

Lastly, QEV's are readily available.

well, the crossman design i'm basing mine off of has just a hammer valve, and I think Ive got a pretty good design, but thanks!