SpudFiles

You can build a variation of the DFTV (http://antennalaunchers.com/dftv19/) that opens when the chamber pressure rises. My copper hybrid uses this, it has been dry-fired successfully a few times but I haven't had time to finish it completely (it cn also be used as a pneumatic and there the valve performs flawlessly as well).
This is the first concept drawing, the design has changed a good bit but the valve is essentially the same: http://home.no.net/galfisk/images/hybrid/hybrid.gif

hmm the design actully vents the piston and does it faster than a tiny pop off, just what I'm looking for thanks