Caliber was a measurement of projectiles used in projectile weapons, such as bullets.
Colt Firearms made .45 caliber guns. (TNG: "Time's Arrow")
In an alternate timeline 1944, Trip Tucker reported that three .50 calibre bullets had pierced the shuttlecraft's hull, asserting that a sensor glitch was not responsible for their picking up old Earth radio broadcasts. (ENT: "Storm Front")