A softer spring would either need to have a wider coil diameter (probably not feasible due to piston internal diameter), have thinner than standard 3.1mm wire (would struggle to deliver enough energy to the piston without massive preload due to the 71mm stroke) or have more coils (with the same free length).

If anyone can persuade Jon to measure the spring in his HW99, perhaps we could find out what rifle it was made for.