Groove Diameter
The diameter measured across the grooves of the rifling, which sets the true bullet diameter a barrel is built to shoot.
Groove diameter is the distance measured across the deepest parts of the rifling, groove to opposing groove, in a finished barrel. Because the grooves are cut below the lands and grooves surface, this figure is larger than the land-to-land bore diameter by roughly twice the rifling depth. It represents the widest internal dimension the bullet must fill.
This measurement is what sets correct bullet diameter for a given barrel, since the projectile needs to engrave into the lands while completely sealing the grooves. A bullet sized to the groove diameter (a .308-inch bullet in a .308-groove barrel) seals combustion gases behind it and rides the rifling cleanly, while an undersized bullet lets gas blow past and strips accuracy out of the system.