Gerry, I'd already cocked, loaded and fired it before your advice. Still, no apparent harm done.
I did know about confining the spring before dismantling, and then letting it expand under control. In my youth, I've taken a few airguns to bits; when I was 9 my parents gave me £1 to go and buy my first air rifle from the local newsagent. Elf'n'Safety was a bit laxer in 1957...
But this is the first airgun I've bought in 45+ years, though I still have my old .177 BSA Cadet Major.
Piston rings, eh? Those sound interesting!