As some of you may know I have been tuning the TX for some years now , so thought I would outline where I have ended up after constant tinkering.
I have been using 21mm diameter pistons for some years with x ring sealing though now I am sealing differently, so : 21mm steel skirted full length piston with synthetic piston ring for sealing, my own aluminium comp tube (with a hard coating that features teflon in the surface ) with a revised breech sealing method . It became apparent that temp shift was an issue for people shooting competition after a friend Rob F had a nightmare with his tx when shooting abroad at temp above 30c . So hmangphil on here and Rob f came up with the Idea of using an engineering plastic for a piston ring, they kindly gave me some material, and I worked with them on producing a ring to work with my set up and comp tube , and the results were impressive very low temp shift and outstanding consistency, Pics below I hope
https://photos.app.goo.gl/6cP8VhmfEzRGjvE46