Jack,
Briefly, the bore size is 12 gauge.
The inside of the 12B shell also measures the same as the bore - .0729”+/-
The difference between the 12B shell and chamber size and the 12A shell and chamber size is that the 12A chamber was designed to accept the paper shells of the day which necessarily had thicker walls than the brass 12B shell. 12A shells were also made in brass for those who preferred brass but the 12A chambers would accept both paper and brass 12A shells.
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__________________
"I'm a Setter man.
Not because I think they're better than the other breeds,
but because I'm a romantic - stuck on tradition - and to me, a Setter just "belongs" in the grouse picture."
George King, "That's Ruff", 2010 - a timeless classic.
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Threaded Mode