I would say you would have to get back into the muzzzle at least 3" to get past the choke. The trouble is you can't use a telescoping gage because you will be compress the gage by the time it reaches the choke. You really need a bore gage, one that has a dial indicator that you push into the muzzle set to a size and then zero then read how many thousands negative it reads. Bore size 12 ga.,.729 (typ), choke .709= modified.
Some have a reading depth of 14-16 inches, good for measuring both ends.

David