Been tuning the tx among other things for around 7 years in that time tried every piston diameter weight stroke configuration you can think of and settled on 21 mm diameter.
Recently got talking to Tony L & Craig p about the possibility of a lower powered rifle for hft , inspired by Craigs excellent Hw 30.
I attempted a 20 mm comp tube a few years back , but couldn't stop it dieseling, so only got 9 fpe consistently, so thought I would have another go at it applying everything I have learned in the years since.
So started with the old comp tube I had( using the tx breech seal set up ) and knocked up a skirtless piston, using an o ring and the hw 30 spring results were promising, so improved the breech with the arrangement I use in my 21 mm , and changed the o ring to a synthetic piston ring, this made power in the tens with 8.4 , but with light 7.3 made 10.8, Tony and Craig would be happy, but I want it for Ft, So aiming for 11fpe plus. Downside is the skirtless piston rocking gives poor consistency, so inspired decided to go the whole hog !
A few random pictures above , in summary I now have a full piston with a steel skirt, which luckily weighs the same as the skirtless one, I will be using piston ring
Aluminium compression tube with bearings to keep everything concentric, breech arrangement that uses an o ring to seal on the outside of the barrel, and seats the pellet,
I have selected a latch rod from my stash that has a longer button on the end to mini mise lost stroke, and increased the scallop in the under lever , managing an extra 2 mm of stroke that's 100 mm.
So latch rod loctite setting as we speak, will be testing over the weekend.
For those wondering I have used ally comp tubes successfully after hard anodising, but for one tube it's too costly, I have the bearings, and piston ring sized so there will be no metal to metal contact, and will be running it with zero lube.