How is the Caliber of a projectile measured?

I always thought one caliber was equal to one inch. So a .50 caliber bullet would have a diameter of .5 inches.

And a 9mm bullet would have a diameter of 9mm. It’s just metric.

johnpowell is correct… .45 caliber shells are .45 inches in diameter. 9mm shells are 9mm in diameter.

shotguns are measured in gauge, where the gauge of a shotgun shell is the number of pellets from the shell which must be lined up next to one another to make a one inch long line. so a 12 gauge shell is filled with pellets that are 1/12 of an inch in diameter.

you guys nailed it, but i like this subject, so i’ll just add that it’s important to be careful with terminology. the main parts of a cartridge are the projectile (or bullet), the casing (or shell), the propellant (or gunpowder) and the primer (which ignites the propellant). everyone here agrees that the caliber is the diameter of the projectile. in many cartridges the casing is ever-so-slightly bigger than the projectile, except in the case of other sporting or military cartridges, such as the .223 Remington, where the casing is significantly larger than the projectile to house more propellant and thus make the smaller bullet go faster and farther.

It all boils down to nobody agreeing on standardizing the method of measurement. Classically American.