SpudFiles

this is my idea i have though of over the last few days at school. the hammer and trigger shape is borrowed from the m14 rifle. The shells are going to be pvc? they can contain anything that forms an air-tight seal in the barrell, for example cone-darts. when the projectile passes the gas return port some of the air is re-routed to the gas return tube, reloading the system. the ejection port is not pictured, but is on the same side as the charging handle. all pieces will be made of duct work metal that is layered,and held together with JB weld.

any suggestions would be much appreciated.

No reason that wouldn't work, the trigger group looks like the M16/AR15 platform.

I'd replace the slide valve with a push button valve from here.

Why not just manually actuate the slide valve by having a trigger attached directly to it?

forgot to add; there is a spring keeping the slide valve closed