Part 2 to this; is it possible to install a choke to cylinder bore only barrel? I imagine it is pretty much just a matter of machining, but could be wrong.
As he said its generally for slugs, or buck shot, where a cylinder bore is common, or improved modified. You'd have to ask the gun smith about your particular barrel. I have an old Sears 20ga semi that was my fathers, and it has a 30'' barrel. I wanted to have our smith cut it down to 26'' and thread it for chokes but he said there is not enough material in the barrel to thread it.
According to the gun, the choke could be rolled into it also. They used to do that before they had the threaded chokes. That or install a Lyman or Poly-Choke. I don't believe they make the rolls anymore, but an older Gunsmith may be able to do it. Also, by cutting off a barrel, the barrel is generally thicker at the 18-1/2" point, so threading one should be no problem.