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In guns, particularly firearms, caliber (or calibre in British English) is the specified nominal internal diameter of the gun barrel bore regardless of how or where the bore is measured and whether or not the finished bore matches that specification. It is measured in inches or in millimeters. For example, a "45 caliber" firearm has a barrel diameter of roughly 0.45 inches (11 mm). Barrel diameters can also be expressed using metric dimensions. For example, a "9 mm pistol" has a barrel diameter of about 9 millimeters. Due to the fact that 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 US customary units, and vice versa.
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. 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.
Good performance requires a concentric, straight bore that accurately centers the projectile within, 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.
While modern firearms are generally referred to by the name of the cartridge the gun is chambered for, they are still categorized together based on bore diameter. For example, a firearm might be described as a "30 caliber rifle", which could be any of a wide range of cartridges using a roughly 0.30 inches (7.6 mm) projectile; or a "22 rimfire", referring to any rimfire firearms firing cartridges with a 22 caliber projectile. However, there can be significant differences in nominal bullet and bore dimensions and all cartridges so "categorized" are not automatically identical in actual caliber.
For example, .303 British firearms and projectiles are often "categorized" as ".30-caliber" alongside several dozen U.S. "30-caliber" cartridges despite using bullets .310–.312″ diameter while all U.S "30-caliber" centerfire rifle cartridges use a common, standard .308″ bullet outside diameter. Using bullets larger than design specifications causes excessive pressures while undersize bullets cause low pressures, insufficient muzzle velocities and fouling that will eventually lead to excessive pressures.
Calibers fall into four general categories by size. Small-bore refers to calibers with a diameter of .32 inch or smaller, medium-bore refers to calibers with a diameter between .33 inch up to .39 inch, large-bore refers to calibers with a diameter of .40 inch or larger, and the miniature-bore historically refers to calibers with a diameter of .22 inch or smaller.
There is much variance in the use of the term small-bore which over the years has changed considerably with anything under .577 caliber considered small-bore prior to the mid-19th century.

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