Bullet Diameter
The actual measured diameter of the bullet, matched to the barrel's groove diameter and frequently different from the cartridge's name.
Bullet diameter is the true width of the bullet across its widest bearing surface, measured in thousandths of an inch or in millimeters. It is sized to match the barrel’s groove diameter so the projectile seals the bore and takes the rifling cleanly, which is why a barrel and its bullets must agree even when the cartridge name does not. A .308-inch bullet, for instance, is built to fit a groove diameter of roughly the same figure.
The number stamped on the box often does not equal the real bullet diameter, because caliber-designation follows marketing and history as much as measurement. A .38 Special uses a 0.357-inch bullet, and a .303 British actually takes a 0.311-inch projectile, so handloaders read the spec rather than trust the name. Matching the correct bullet diameter to the caliber of the barrel is a basic safety and accuracy check before any load is assembled.