Pistol calibers refer to the diameter of the bullet used in handguns, measured in either inches or millimeters. Common calibers include .45 ACP, 9mm, and .40 S&W. Each caliber has unique characteristics, affecting factors like recoil, accuracy, and stopping power. Different calibers are suited for various purposes, such as self-defense, target shooting, or law enforcement. For example, the 9mm is popular for its manageable recoil and high capacity, while the .45 ACP is favored for its stopping power. Understanding these differences helps shooters choose the right caliber for their needs.