I bought a cut off Polychoke, with the barrel stub still attached, as I had a SS shotgun with a barrel ID over .730, which is the cutoff size for screw in chokes. I think I paid about $15 for the Polychoke. They come in several different thread diameters, tailored to the OD of your shotgun barrel. I think they all are 36 TPI.
I tried every which way to get that barrel unscrewed. Kroil and heat failed to budge it. I ultimately chucked it in my lathe, and with a boring bar, gradually reduced the ID, until the remaining barrel threads collapsed, and fell out, leaving the choke threads undamaged. I had to turn my shotgun barrel down quite a bit at the muzzle, to match up to the small diameter size of this particular Polychoke. I added the threads to the barrel, and screwed the choke on. I had to adjust the shoulder on the barrel to get the choke to clock right. Each time I screwed it on it was crooked, to the same side, unbelievable. I know I threaded it straight. Anyhow, I made up a mandel, fitting snugly in both choke and barrel, and ran it up. It was straight then. Some red loctite sealed the job.
I took it out back to shoot it, and it patterned low. About a foot at 25 yards. It was due the the front bead on the choke being raised higher than the original bead. I asked what does Polychoke do on their installations? I found out, they bend the barrel. I went off to the 12 ton hydraulic press, made some wooden blocks to support the barrel, and bent it upwards. The pattern is centered now. Job complete. With a .742 barrel ID, I can get some super tight patterns, being squeezed down through the choke, if I want, or nicely spread ones for the closer targets, just by twisting the choke. I think I can go down to about .680. That's .062 restriction. The gun is an old 12 ga Stevens, that had a damaged barrel. Too big of a barrel ID for screw in chokes, so an external one was a practical solution. It makes a wonderful truck gun.