I actually measured the 1300 and 1322 Pistons this afternoon as I realised that you can do it without stripping the gun, the 1300 piston is 47mm long, and the 1322 is 70mm long.
So you need to find a flat top 1322 piston that can be shortened by 23mm, I did have some Krusty's Kustoms flat top Pistons I was going to measure for you but they seem to have vanished from my spares box.
What I did find though is that I'd already machined a brass O ring piston head for my 1300. :-
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All the best Mick