i had a similar idea but using a single main storage chamber (4"pvc) a smaller 2" firing chamber pneumatic piston valve actuated with blowguns and a 3/4" barrel and bolt action breech loader housed in a 1" outer barrel. but i think the pressure of the main chamber will decrease to much to pressurize...

use a gate style valve (example:blowgun) they make larger ones not sure on pressure rating though

the chamber is an unexpanded bottle and a mini mag lite the barrel is just an ink pen that fits a pellet in it

made this mini a couple of nights ago grill ignition and butane fuel barrel fits a .177 pellet perfect