My original idea was to have the performance of the QDV with the added bonus of having semi automatic operation.

@veolcity: Can a spring not work in compression and extension?

In the ready to fire position, the spring will be in tension, enough to pull the piston/valve/spool enough to allow the chamber to vent into the barrel. The air will accelerate both the projectile and the piston. The piston will ...

@POLAND_SPUD: You could stick another spool on the rod which would block off the air supply. cos aren't spool valve usually used for this kind of thing?

Isn't that what a QDV is? It seals the chamber.
I thought that piston valves generally have a sealing face and have a pressure differential across them which is utilised to open them.
QDV is a spool valve which, when closed has no forces (kinda) on it.

is that about right?

@kjjohn: Does all of the ...

This is just a random idea that popped up into my head. I decided to stick it up here for criticism/approval before I go to sleep.

It is a really crappy drawing and thus I apologise in advance.

It is just a normal qdv but with a spring!
When the valve is closed, the trigger (orange) holds the ...