The calculated bore dia of a 4 gauge gun is 1.052" This is also the diameter I have seen listed in ever bore size chart from the various proof houses. A gun with a smaller bore diameter, (Common on breech loading guns) even if chambered for a 4 gauge shell is not proofed as a 4. 5 gauge diameter is .976" while 6 gauge is .919". My understanding is that most 4 gauge breech loaders will have their bores marked as a 6. I have a Birmingham proofed 12ga which has its bores marked as 14 (.693"), but if one asked the Birmingham proof personnel what size is a 12 gauge they would tell you .729" & that would be the correct answer.
the bore size of any individual gun is what the maker made it.
I recall reading some time ago that Buck Hamlin took a decrepit L C Smith 12 ga damascus & cut the chambers for a 3˝" shell & then began loading it up. As I recall he used loads of Blue Dot powder & loaded both powder charges & shot loads well above anything found in the loading manuals. He ceased his experiment when the rotary bolt would no longer stay closed but was blowing open with every shot, but did not succeed in bursting the barrell. I don't recall if he had recorded the pressures of these loads or not.
