Measure the outside diameter of your barrel at the spot you plan to cut it. An OD of at least .845” is required for 12 ga Rem-Choke tubes, .825” for Tru-Chokes, or .805” for Tru-Choke “Thinwall” tubes.
All these minimum diameters are based on the bore being concentric with the outside of the barrel. If the bore if found not to be concentric then your gunsmith will have to measure the actual barrel walls to determine if adequate thickness is there for safe threading..
Hardships are quickly forgotten. Intense heat, bitter cold, rain and snow, fatigue, and luckless hunting fade quickly into memories of great fellowship, thoughts of beautiful country, pleasant camps, and happy campfires.
Fred Bear 1902-1988