Bowkett used to do many conversons on the HW35 in the 1980s. They went against current trends but they were in high demand and paid his mortgage so he must have done something right. He told me that he used a 4 mm port, lined the piston with a thick steel sleeve, fitted a steel top hat and used one of his universal springs made for him by a chap called Mike wade. These were 24 coils and of much smaller outside diameter and wire than standard. The piston was also fitted with his piston head. The result after being "balanced" was a very soft shooting full power rifle, the .22" versions had to be de-tuned, and amzingly quiet.
Just goes to prove there are more ways than one of skinning a cat