I've got a 12-ga barrel with a factory choke and a 10-ga barrel with a custom Briley choke and a couple factory chokes.
The factory choke from my 12-ga field is 1.5 inches long overall. The smooth, strait, un-threaded section at the muzzle end is about 0.625" long by 0.846" dia. The threaded section is about 0.75" long with an outside thread diameter of 0.831" and I am not sure but think it could be 40 threads/inch; my reading glasses are not powerful enough to tell for sure. The smooth. strait, un-threaded section at the breech end is about 0.125" long by 0.802" diameter. I would like to have Briley make a custom choke of a much longer length.
The Briley choke from my 10-ga field is 3.5 inches long overall counting the 1.0" long knurled section outside the barrel. The 2.5 inch long portion of the chioke inside the barrel is as follows: The smooth, strait, un-threaded section at the muzzle end is about 1.55" long by 0.876" dia. The threaded section is about 0.80" long with an outside thread diameter of 0.870" and again, I am not sure but think it could be 40 threads/inch. The smooth. strait, un-threaded section at the breech end is about 0.15" long by 0.860" diameter. I've also got an original factory choke and a factory choke which Briley reamed out to "open" for shooting slugs, but did not measure them.
As far as I know Thompson-Center only had "full" choke tubes for either 12 or 10-ga. I guess it must have been less expensive the to make a barrel and add a choke tube than to make a barrel with a integral choke.
