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'B' bore (measured at 9" using a plug gauge) and 'M' muzzle. 14 = .693"
Fractional bore was not used until 1887 Rules of Proof so 12 = .729" - .750"
post-1887 12/1 would be .740"
I can't explain why the barrels are marked CHOKE rather than NOT FOR BALL
NITRO PROOF and the shot charge started in 1896; occasionally MAXIMUM, MAXm or without MAX

Reproved after 1925 (chamber length indicated) 20g with B & M on the barrel

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