Standard McMaster Carr manual ball valve / globe valve operated by a 
Hitec servo.
About 0.5 seconds operating at 7.2V at 1.5A if I read page correctly.

    http://www.rasdoc.com/paul/rockets/ballvalve.html

Operates happily at 1500 psi.
Maybe not quite fast enough for yer potato cannon - but maybe it is 
:-).
____________

Servo is super grunty 400 inch-ounce (0.3 kg-m)
Titanium gears
About $US115
        http://www.hitecrcd.com/Servos/hs5995TG.htm

Servo gearboxes.
Nice idea.
Takes standard hobby servos and multiplies torque (and travel time) by 
N - typically 5 times
Reasonably priced considering.

        http://www.servocity.com/html/robotzone_servos.html


 

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