What does caliber measure in firearms?

Caliber in firearms refers specifically to the bore diameter, which is the internal diameter of the barrel of a firearm. This measurement indicates the width of the projectile that the firearm can chamber and fire effectively. It is usually expressed in either millimeters or inches, depending on the system being used. For example, a firearm that has a bore diameter of 9 millimeters would be referred to as a 9mm caliber firearm.

Understanding caliber is essential for numerous reasons, including the selection of ammunition that is compatible with a given firearm, as well as for identifying the performance characteristics of different firearms. Smaller calibers generally fire lighter and faster rounds, while higher calibers tend to have larger, more impactful projectiles suitable for different shooting purposes, ranging from sport shooting to hunting.

The incorrect options do not accurately represent what caliber measures within the context of firearms. The length of the barrel refers to how long the firearm is, while the weight of the bullet pertains to the mass of the projectile itself, and the firing distance relates to how far a firearm can effectively shoot. None of these options provide insight into the bore diameter, which is the fundamental definition of caliber.