After cutting down the barrel of my old French MAPF .22lr semi-auto pistol to 3.8" recently to re-thread it 1/2"-28tpi, I found that the bore at the new crown was significantly low and left of centre as related to the OD of the 1/2" barrel. The can I made for it being 5.5" OAL, and using just the new threads to register on the bore, this made for a fussy job. I chucked the barrel in my little lathe, put a brass rod into the bore with the last inch turned down to just barely squeeze into the barrel, then proceeded to adjust the 4-jaw and add a couple of shims here and there until the alignment rod was turning perfectly true. This made the breech end turn in a very wobbly way when I spun up the lathe but the muzzle was perfectly true, the brass rod sitting still throughout its length.
With that done, threading was pretty easy, just maintaining square with a 1/2"-28 die and using a wrench and some oil to slowly advance, test spin, back off, advance another 1/2 turn... until it was threaded for about 0.6". The threads 'dig in' visibly by their finish point on one edge of the barrel, are a bit flat-topped and shallow on the opposite side. When the can is snugged up tightly and I insert the brass rod, there's absolutely even spacing all the way around it at the 0.275" exit hole in the cap. No baffle nor end cap strikes. Thing shoots accurately, and considering it's only got 4.5" of baffled volume with 8 K baffles, it's pretty darn quiet, easily comfortable on my overly sensitive ears in an empty small room.
This isn't a clamp-on suppressor story. But I say it because it demonstrates one example, among several in my experience of crookedly drilled bores, that barrel OD often has little relation to bore axis. I've had a Chinese air rifle which had the bore perfectly centred initially at 18", but when cut down to 10" (seeking, and finding as calculated, it's proper length for optimum efficiency of desired air delivery to velocity ratio, reducing the role of barrel friction nicely while rendering a good back yard squirrel gun with many more shots per air fill) the bore was almost 1.2mm off centre. The hole had a big arc to it. I have no idea how the manufacturer managed to get the drill back on centre, but I had to do some tricky work to suppress the thing with a 10" long K baffle can without baffle strikes. Worked out well, but a clamp-on can wouldn't have been workable at all.
