My first idea was to use something flexible. You've got air pressure working for you, so something rubber would hold onto the barrel until it was pulled completely off, well after the trigger part (brass) was far enough back to block air flow. Then the air pressure that's only in the trigger part would push it back over the barrel, and since it's flexible it would push back into it until the air could re-load the "expansion chamber" (sorry, making up terms).
You've probably already thought of this, but would you try using one of those rubber springy things? The little quarter machine poppy domes would work perfectly, if you could get them fastened well.
::edit:: BLAST! I could swear I put this in homemades... can I get a thread move please?
Edited by pinhead52, 11 August 2005 - 01:02 PM.