I just made a barrel for mine from a crosman 1322 barrel (around 12mm OD). An extra 4" seems to be optimal in terms of velocity and not being too cumbersome. I'd suggest finding a suitable blank, then just get a machinist near you to knock one up. I reduced the lead in "cone" by about 0.75mm diameter, to save a bit of gas. It feeds prefectly, so I'd say it can only help with efficiency. As you would guess I only redcued the OD for the section that fits inside the original frame, leaving the protruding 4" at the full OD. It's gained 100 fps and is more accurate too.
Ref the rail, some weaver bases fit straight on; just watch they don't wrap around to much and foul the pellet pusher on the mag
HTH - JB
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.