I was planning on drilling out the threads on the original barrel bushing/oring and attaching it in place with a set screw from the top (my gun already has a threaded hoile for that) so the bushing/oring and barrel can be removed independently.
It'll be a .22, with a 450mm long barrel for now, though I aim to make it easily replaceable to allow a shorter one if I want to switch between them. Mainly I didn't go .25 because I felt the gun may be a bit underpowered for long range accuracy with heavier pellets, though now I'm starting to doubt that will be the case.I also drilled out every part between the air tank and valve (so the gun is currently unregulated), so the plenum size will be 2-3 times bigger at least and the flow restriction is massively reduced. Before, all airflow from the tank to the valve was restricted by a .5mm hole as well as the very restrictive reg, but now it's 6mm at the smallest point. I'll be adding a huma regulator as well eventually, but that's later on once the gun is ready to shoot.
so you would have more than enough power for 25.. mines doing 19.6fpe with a 9 inch barrel..im betting with the added plenum you now have ,, it would be easy to get 35 fpe with a 18inch barrel.