Awesome stuff!

I once tried to model how much friction would be needed in the last inch of travel to make a difference to the peak recoil, and I forget the numbers now, but it was a lot!!

I'm fairly convinced that the parachute seal feeling softer is probably just because it is creating a better seal, rather than adding enough extra friction to slow down the piston at peak pressure.

I wondered about adding a tiny choke/taper at the end of the comp tube, but when it came down to it I think to have any noticeable effect the forces involved were so large that the piston would have just got stuck there and made it imposible to cock the rifle!

I've always fancied the sliding weight idea. I'll try to add that into my model to see how much mass you need and how critical the spring rate is in terms of timing. I think one issue will be that to have a significant effect the added weight will need to be comparible to the mass of the piston, so kind of like doubling the piston mass. You can't get away from the fact that more moving mass has to equal more movement/recoil, but if you can delay the movement until the pellet has gone then who cares!