Jon I have just looked at your spec above I have realised it's very close to to my spec for the HW77/97 except for the piston weight where mine is around the 250gm all in.

What pre-load & spring dimensions are you using? I like to work around 40mm in general; I have a 3 springs I can use depending on shot cycle required but one of which is only suitable for .22.

I'm in the process of having new piston heads CNC machined to take HW seals for the TX, they will screw straight onto my latch rods.