Kem:

18.5 is the bore measurement in millimeters. It works out to 0.728 and a fraction inches, or slightly less than -.001 under
nominal 12 gauge bore measurement of 0.729 inches. 18.3 would be .720 and a fraction inches, giving you a .008 choke constriction. 17.2 is roughly 0.677 inches, giving you a choke constriction of .052 inches or seriously extra full.

Rem