Bore Diameter
The diameter measured across the lands, the raised ridges of the rifling, which is smaller than the groove diameter of the same barrel.
Bore diameter is the distance across the bore measured land to land, meaning across the high points of the rifling that protrude inward. Because the lands and grooves alternate, this land-to-land figure is always smaller than the groove diameter, and the difference is roughly twice the rifling depth. A .308-bore barrel, for example, measures about .300 inch across the lands.
Bore diameter is the number many cartridges are named for, which is why a caliber designation does not always match true bullet size. The lands are what actually bite into and spin the bullet, while the deeper groove diameter sets the size the projectile must fill to seal the gases behind it.