What's an equivalent caliber for a 12-gauge shotgun slug?

Do you mean by weight or diameter? In American usage “”12″ gauge is .729 inch diameter. Shotgun Slugs are usually sized smaller so they can pass through the choke constriction without damaging the barrel. read more

A 12 gauge bore measures 0.729", slugs for smoothbores are sized smaller (.690") so they can clear the choke. Caliber can refer to the diameter of the projectile, or the measurement of the bore between the lands or between the grooves of the rifling. read more

There was an old guy that went hunting with my grandfather that referred to the bore size in inches when talking shotguns. The 12ga was a 3/4 bore, the 16ga was a 2/3 bore and the 20ga was a 5/8 bore. read more