You could even take it a step further

and make it an air-conserving valve with adjustable timing. I designed one like this a few years ago. Probably lost my sketches since then.

[IMG][/IMG]

In this pic, the blue cylinder is a lightweight piston with a non-airtight fit in the valve housing. When the valve opens (from a backwards hammer strike or maybe blow-open) the blue piston gets blown forward until it hits the shoulder on the valve needle and closes the valve, like the displacer piston from Steve's old DP392. After the valve is shut air leaks past the piston and the spring resets its position. The air inlet (not shown) from the pump or extended reservoir is behind the blue piston.

That gives you a volume-metered air conserving valve for operating at a repeatable pressure. The volume of air dumped could be adjusted by cutting a thread on the spool. Having a moveable stop for the piston or it's return spring ride on those threads would turn the valve stem (the part sticking out the back of the housing) into a power-adjusting knob. Just turn the valve stem to adjust how far the piston has to travel before it closes the valve.

I THINK it would also be self-regulating in a pumper, settling down to a predictable pressure if you left the power setting alone and gave it the same number of pumps every shot.