Finally got round to this... think of a meteor buffer washer on steroids
Completely different securing method though, gives 2mm of carefully controlled travel, but I don't think it uses it all (which is the way it should be).

Idea being to soften the landing of the piston after it's second forward stroke.

First impressions are that it's working, but the gun is still settling so need to tune a little first.
Not a massive improvement, but noticeable, for sure. A more pliable bushing material may help too - it's currently using a damper bush from a landcruiser

Will bring it to the bash so other folks can try it, as it might all be in my head