how are bullet calibers measured

Caliber - Wikipedia, the free encyclopedia


** Caliber **

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"Calibre" redirects here. For other uses, see Calibre (disambiguation).

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From left: .50 BMG, .300 Win Mag, .308 Winchester, 7.62×39mm,
5.56×45mm NATO, .22LR
A .45 ACP hollowpoint (Federal HST) with two .22 LR cartridges for
Side on view of Sellier & Bellot .45-cal ACP cartridge with a metric ruler
for scale

In guns, particularly firearms, *caliber* or *calibre* is the approximate
internal diameter of the barrel, or the diameter of the projectile it
fires, usually shown in millimeters, or in hundredths or thousandths of an
inch. When expressed in inches in writing or print, it is shown in terms of
a decimal fraction: .45 caliber, for example. When the barrel diameter is
given in inches, the abbreviation "cal" can be used. For example, a
small-bore rifle with a diameter of 0.22 inches can be called a .22 or a
.22 cal; however, the decimal point is generally dropped when spoken,
making it a "twenty-two caliber" or a "two-two caliber" rifle. However,
when caliber is expressed in millimeters, this is noted, as in, "9mm

In a rifled barrel, the distance is measured between opposing lands or
grooves; groove measurements are common in cartridge designations
originating in the United States, while land measurements are more common
elsewhere. Good performance requires a bullet to closely match the groove
diameter of a barrel to ensure a good seal.

While modern cartridges and cartridge firearms are generally referred to by
the cartridge name, they are still lumped together based on


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