The 5.56 caliber refers to the 5.56x45mm NATO cartridge, commonly used in AR-15 rifles. The number 5.56 denotes the bullet’s diameter in millimeters – it’s 0.224 inches.
Caliber measurement can vary based on the country of origin and the historical context in which the ammunition was developed. For instance, calibers like 5.56mm and 9mm are measured using the metric system, which is standard in many parts of the world, particularly in Europe. On the other hand, calibers measured in inches, often denoted with a dot, like .308 or .22, follow the Imperial system of measurement, which is more common in the United States and the United Kingdom. The dot representing a decimal point indicates the caliber is in inches—a .22 caliber bullet has a diameter of 0.22 inches.
However, keep in mind that the caliber name doesn’t always match the bullet diameter. For example, the .223 Remington has a bullet diameter of 0.224 inches, and the .38 Special has a bullet diameter of 0.357 inches.