As far as I know choke is removed to install all choke tubes in a gun making it cylinder bore. It is then internally threaded. The typical choke tube installed is often 1 1/2" long. CZ and Krieghoff have 1 1/2" long choke tubes in their 12g. guns and they perform quite well. A lot of makers use 2" long tubes.
I have had it done two pump guns and it worked very well.
Not all guns have the "meat" at the muzzle to install tubes though. Mike Orlen used to have a pricing sheet that spelled out the dimensions needed before he could install them.
