From my old Webster's New World Dictionary for the relevant part; Bore, Noun; 1. A hole made by or as by boring. 2. (a) The hollow part of a tube or gun barrel (b) Its inside diameter: Caliber

Gauge; relevant definitions Noun 1. A measure; standard measure or scale of measurement 2. Dimensions; capacity; extent 6. The size of the bore of a shotgun as determined by the number per pound of spherical projectiles fitting the bore

From definition 2b of bore it would seem it would be correct to say a gun was a .729 bore, as to whether that would transfer to a 12 bore may be debatable, but definitely correct to say it is a 12 gauge, from definition 6 of gauge.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra