A firearm caliber refers to the internal diameter of a gun barrel and the corresponding size of the ammunition it uses. Calibers are typically measured in either inches or millimeters, with common examples including .45 ACP and 9mm. The caliber affects the bullet's size, weight, and velocity, influencing its performance and suitability for various shooting activities. Different calibers are designed for specific purposes, such as target shooting, hunting, or self-defense. For instance, small caliber rounds like .22 LR are often used for training and small game hunting, while larger calibers like .308 Winchester are favored for big game hunting and long-range shooting.