14ga measures to .693" (17.6mm). Many Full choke 12ga guns have a choke dia of .690" (17.53mm) or slightly less. It is not at all uncommon to see 12ga choke bbls under early proof rules marked 12B/14M or a 12ga bore with 14ga choke dia.

14 3/4 as a gauge is smaller than a 14ga & as such would measure .680" (17.27mm) if true to size. Standard for a 12ga bore is of course .729" (18.5mm).

Miller/TN
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