You could maybe go for a slightly larger O-ring. Just need to find out what size the current ones are then get one that's half a mil thicker to see if it helps.

What would you say is the best way to go about smoothing the cocking slots out? On my Cometa I used a dremel, but I'm not sure i'll get in with it as they are so small on the HW45.

Did you just go for another HW piston seal or an aftermarket one?