Thanks for this interesting thread.
Here is how I installed liner in a 12 bore shotgun, which was pitted beyond anything I would shoot.
I used a standard 3/4" chucking reamer. Drilled a piece of brass rod, slipped it onto the shank up toward the flutes and soldered it in place. Put it in the lathe and turned it to bore diameter, to make a fitted pilot. Turned down the end of the shank and fitted a sleeve into which the extension rod was fitted. Soldered this assembly together. Using a variable speed electric drill, started pulling the reamer through from the muzzle - couldn't start at the breech, the reamer would have cut the tops off the threads. Lubed, reamed, cleaned over and over until the reamer just reached the threads. Even at .750, there were still some pits.
Used 4130 Cr-Mo seamless aircraft tube to make the liner, .750 OD, just over .650 ID. Machined the liner to match the nose of the breechplug. Made sure I could slide the tube into the reamed bore. Degreased everything, plugged the liner, applied epoxy (IIRC, JB Weld) to the liner and the bore, pushed the liner into place, a bit too far. Cleaned all epoxy from the breech area, removed the plug from the tube. Screwed the plug in, forcing the liner foreward a bit - I wanted the liner hard against the face of the breech plug. Plug was waxed. After the epoxy was cured, I turned the liner flush with the muzzle.
Proofed the barrel.
There is enough meat in the liner that it could be jug choked.
If I were doing this over, I would have used a bit thinner liner, just to keep the barrel a bit lighter.
Used the same rig to ream out another pitted 12 bore, although I had to adjust the diameter of the brass pilot. This one reamed smooth, all pits gone, nice and smooth. I will still line it though, the walls are a little on the thin side. Going to use thinner liner stock on this one.
First gun was a respectable Birmingham gun in nice condition externally; second is rougher. Needs more restoration in addition to the bore. But both would have been wallhangers without sleeving.
Cr-Mo tubing can be obtained from .aircraftspruce.com or .ca depending on where you live. Cost is very reasonable.
Who is TJ, maker of rifled liners?
Here are shots of the installed liner, and the gun.