Thanks for the tips gents.
The gun was sold to me as having an ox spring. When I opened it up it did have preload, and I thanked the compressor I'd built for myself!
I suspect the tight fit is either grime, or the piston head. Of course, a tight piston as Garvin says is an option too. But I think , as Johnj says, I'll give everything a good clean and see how things go.
I have a Maccari old school kit that I hope to install. The spring is short, so I have a feeling it might be installable without a compressor. Best to keep it around though!
Ginb AA Prosport 0.177, Ginb FT V-Glide Hw77k 0.177, CS 500 V-Glide Hw80 0.22, Steyr LG 110 0.177, Gamo Compact 0.177, Hw45 0.177, Beretta 682X Trap 12b, Beretta 682 Sporter 12b, WH Pollard hammer 12b