I have an early breechloading hammergun with Birmingham proofs and marked "13". In spite of the "13" marking, the bores measure .729 and .722 and the chambers fit standard 12 gauge RST shells. I am guessing that the gun may have originally been chambered for 12b shells with smaller bores to match and then re-chambered for 12a shells, when they became standard, with a corresponding enlargement of the bores. Can the transition from 12b to 12a shells/chambers be dated? When were the first and last 12b breechloaders made?
