Certainly worth looking into, though I'll try a few single shots to see what the power would be like before making anything.
edit: put a few single shots (barrel hooked up directly to the pop-off valve without the blow forward breech) and got an average of 240 feet per second for 1 gram lead pellets and 360 feet per second for 0.25 gram airsoft BBs. It's not that bad in terms of power, in the sense that it would make a very decent skirmishing project, and at 2 ft/lbs with lead pellets would make a reasonable plinker, but I'm looking for a bit MOAR POWER! - so on to the pop-off piloted piston valve it is
![Smile :)](./images/smilies/icon_e_smile.png)