large-bore refers to calibers with a diameter of 0.40 inches (10 mm) or larger.

medium-bore refers to calibers with a diameter of 0.33 inches (8.4 mm) to 0.39 inches (9.9 mm).

small-bore refers to calibers with a diameter of 0.32 inches (8.1 mm) or smaller.

miniature-bore historically refers to calibers with a diameter of 0.22 inches (5.6 mm) or smaller.

Good performance requires a concentric, straight bore that accurately centers the projectile within the barrel, in preference to a 'tight' fit which can be achieved even with off-center, crooked bores that cause excessive friction, fouling and an out-of-balance, wobbling projectile in flight.Ĭalibers fall into four general categories by size: Measurements 'across the grooves' are used for maximum precision because rifling and the specific caliber so-measured is the result of final machining process which cuts grooves into the rough bore, leaving the 'lands' behind. In a rifled barrel, the distance is measured between opposing lands or between opposing grooves groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere in the world. Since metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in non-metric units, and vice versa. For example, a '9 mm pistol' has a barrel diameter of about 9 millimeters. Barrel diameters can also be expressed using metric dimensions. For example, a US '45 caliber' firearm has a barrel diameter of roughly 0.45 inches (11 mm). In the United States it is expressed in hundredths of an inch in the United Kingdom in thousandths and elsewhere in millimeters. It is measured in inches or in millimeters. In guns, particularly firearms, caliber (or calibre sometimes abbreviated as ' cal') is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bore is measured and whether the finished bore matches that specification. Side view of a Sellier & Bellot 45-cal ACP cartridge with a metric ruler for scale

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