One other thing I didn't mention. It is possible that on an early gun that the chamber is for a 10B brass shell. During the early period they were made for either 10B or 10A shells. The 10B was the first chambering and made for thin wall brass shells that had an inner diameter close to bore size and thin walls. The 10A was a later brass shell with the outside diameter of a paper shell and a larger inner diameter. The early back action I own had only one factory record, being that it was returned around 1900 to have the chamber enlarged from a 12B to a normal 12 gauge chamber. If a modern 10 gauge shell of the proper length will not chamber, you could have a gun with a 10A chamber.
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