I'm working on the theory that 100mm of compressed spring room should put me in the right ballpark power wise, so the bit inside the piston is just to take up 10mm of the 120mm of spring room inside the piston and to add weight to the static part of the gun.
Effectively this piece takes up the space of 5 Dowty washers (which I'm running low on), if I find that I need more Spring room I can simply machine that 10mm piece off.
Another spring that I'll be trying out is a 25 coil HW99 Spring which will need even less Spring room than I have now --- basically I'm just leaving all my options open at present.
All the best Mick