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It's the inside diameter (measured in "gauge") at the bore (B) and at the muzzle (M).
The bore measurement is usually taken at a specific distance from the breech end of the barrels, but I don't recall what it is. So, the 12b/12m is clylinder bore (no ckoke), and the 10b/11m would be approx. 0.025" constriction.
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