I almost put my Bob Long Spyder into that competition... it used regulated air to push the piston bacwards from the front of the gun, and a forward bias on the piston to shoot it. 7ms operation, vent to fire, and fairly easy teardown. I did have a spring return system to trap the air, but no cutoff. That was timed it.
Sounds like the design I filed with the patent though. You were on exactly the same path, with Andy and the Husher also.