Thanks guys....the .22 was a bit of a task and risky too. The barrel on these, coming pinned to the one piece trigger guard/ cocking lever, is probably worth in excess $300 all on its own....even if you could locate a spare. Getting it wrong here would have been quite traumatic...
I had to set up the barrel in the lathe using a shell of Nylon. Had to bore out the lump of Nylon to the same slight angle of the O/D of the barrel. Then split it in half with a slitting saw in a Milling machine. The 2 halves could then be clamped around the barrel.
I was then able to clamp this solid lump into the chuck jaws, without marking the precious barrel..
Next step was to drill out the OEM .177 barrel liner to 8mm dia. Drilling from both ends, I luckily met in the middle of the barrel, without disaster and everything looked concentric to within 0.1mm.......Needing a stiff Whiskey, I rested down from the project the following day. (6hrs work so far)
I then took some stock .22 Walther barrel from Chambers and machined it to 7.9mm dia. I got an error of 0.1mm end to end, which was not bad, but polished the 8mm dia end until I achieved a perfect 7.9mm diameter along the whole length. I had to achieve this as i was aiming for the barrel to slide into the 8mm bore I had drilled, but without any hint of a drive fit, which could potentially crack the valuable outer casing if i had to hammer it in due to any tightness. (3 hrs) ...left the work alone for another day.
The next day, I machined the groove for the breech O ring, crowned the barrel and checked for any burrs. (2hrs)
Finally, I pushed the barrel into the outer casing with some 638 bearing fit. Left it to set for 24 hrs and tested.
.22 JSB Exact put 5 pellets into a 1 inch circle at 10 yards and hit 397 fps ...A real rat cannon at that sort of range, but I noticed the pellet starting to nose dive at much after 15 yards or so, striking a good inch low and 2 inches at 20 yards. The .177 is dead flat across the first 20 yards so that is the gun I use most. Even so, still a very satisfying project and one of a kind..