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!