Caliber Designation
The naming convention used for cartridges, which often does not match true bullet diameter, as a .308 is 0.308 inch but a .38 fires a 0.357.
Caliber designation is the name a cartridge is sold under, and it usually combines a nominal bore or bullet figure with a maker, a parent case, or a year. Sometimes the number is honest: a .308 Winchester fires a 0.308-inch bullet, and a 6.5 Creedmoor uses a 6.5 mm projectile. Often it is not, because the name was chosen for tradition or marketing rather than precise measurement.
The classic traps are the ones where the designation and the real bullet-diameter diverge. A .38 Special and .357 Magnum both fire a 0.357-inch bullet, the .38 figure being a leftover from older case dimensions, and a .303 British uses a 0.311-inch bullet despite its name. For this reason the standardizing body saami publishes exact dimensions for each caliber, and shooters check those specs rather than reading the headstamp literally.