What is the term for the inside diameter of the firearm barrel?

The term for the inside diameter of the firearm barrel is referred to as the "bore." Understanding the bore is essential for several reasons, particularly in relation to the type of ammunition that can be safely fired from the firearm. The bore size greatly influences the ballistics and performance of the projectile, as it must match the caliber of the ammunition designed for that specific firearm.

The other terms listed have different meanings; the "breech" refers to the rear part of the barrel where the cartridge is loaded, and "barrel" itself is the entire tube through which the bullet travels when fired. The "action" pertains to the mechanism that loads, fires, and ejects cartridges, which is separate from the physical dimensions of the barrel itself. Recognizing these distinctions helps in properly understanding the components of a firearm and their functions.