Why not use an electrically powered solenoid valve then to eliminate this inconsistency?

One: It needs a battery then. More parts, me no likey

Two: It has to reset somehow. Leaving the button/trigger pressed would leave the setup powering the solenoid.

If nothing else works, i could make a ...

I need a valve that dumps pre-set amount of air out with a single push of a button.

What comes to mind is a 3-way valve. Air in thru port1, out thru port2, pressu button and port2 vents out to port3.

BUT... There is a problem.

The valve i have is quite sensitive to how much you push the button ...

Planning on building a "bolt-action" sniper-rifle using existing HPA-rig i have...

What i need is a solenoid for the air.

Would this work:
http://www.ebay.com/itm/1-4-2-Way-Normally-Closed-Pneumatic-Aluminum-Electric-Solenoid-Air-Valve-12V-DC/182078724061?_trksid=p2047675.c100005.m1851&_trkparms ...