what you need is an inertia driven transfer port tube thingy that on full extension actually comes thru into the barrel the depth of the transfer port.

That way you get maximum compression from the piston, as it stops dead the extension tube shoots forward, being hollow to the depth of the transfer port it captures all the air it is holding and further compresses this air as it shoots forward into the transfer port. The trick would be a slight taper going to parallel to it self centres, o ring round its base so it seals into the transfer port or have an O ring set into the transfer port at the barrel end or just have a breech seal that seals onto the outside of the extension tube.

If you were clever with the design of this you could make it to it was a surge damper also ..Like a harper valve on a springer LOL