Got it kinda sorted today. Sorted enough to have a few shots and chrono, not sorted enough to be happy.
Put all the parts in at Urban Assault today, Prometheus hard rubber included. Few shots through the chrono: 570-590 FPS. Shooting high, which I'm not too fussed about as I can solve that easily, the inconsistency is annoying though.
Didn't have time to do anything else to it though.
Took it home, took it apart, lubricated the cylinder and piston, cleaned the barrel out. Did a few test shots, which is where I encountered a few problems..
The hop rubber (Prommy hard) was too hard. The lip that stops it sliding completely on the barrel stopped the BBs, like it should do, for the cylinder head to push them into the chamber when I push the bolt forwards. As it's a hard rubber, the BBs literally jammed here. The rubber was so hard, that I had to slam the bolt forward (not a good idea, but I couldn't get it apart or anything as it was partially cocked). When I took it apart, it had damaged the BB badly, and ripped the hop rubber slightly.
I thought I had the rubber on wrong, took it off, re-assembled the rifle - same thing. Thought I'd risk taking a shot after forcing the BB into the chamber - it was jammed so badly and the air seal so good that it stopped the piston from reaching the back of the cylinder head.
Put a MadBull blue hop rubber in it (soft rubber), and no jamming issues whatsoever. Haven't chance to test FPS or accuracy/range as it got dark.
Any ideas on the hard rubber? As I'll be needing it soon for the best air seal possible. Otherwise may have to try a PDI W-Hold Bucking.
Another thing, when I shoot (a BB, not dry firing) the rifle 'rattles'. It only happens when I shoot, I thought it was lack of barrel spacers (was only testing at first and the spacers were hard to put in the outer barrel), but now I've added spacers, it's still the same.
This happens with any weight BB (from .2 to .4). I think it used to happen when I used .2s at 500 FPS. Could it simply be the high FPS causing it (.4s still travelling at 415 FPS, whereas they travel at 355 FPS when 500 FPS with .2s).
I'll upload a video tomorrow to show what I mean about the sound.
Any ideas guys?