Caliber - Metric Versus Imperial

Metric Versus Imperial

The following table lists some commonly used calibers where both metric and imperial are used as equivalents. Due to variations in naming conventions, and the whims of the cartridge manufacturers, bullet diameters can vary widely from the diameter implied by the name. For example, there is a difference of as much as 0.045 inches (1.15 mm) between the smallest and largest of the several cartridges designated as '.38 caliber'. And it may be noted that .38 inches is more than 9 1/2 mm.

Common calibers in inch and their metric equivalents
Inch caliber Metric caliber Typical bullet diameter Common cartridges Notes
.20, .204 5 mm 0.204 in .204 Ruger, 5 mm Remington Rimfire Magnum
.22 5.56 mm 0.223-0.224 in .22 Long Rifle, .222 Remington, .223 Remington, 5.56×45mm NATO, 5.7 x 28 mm, .22-250 Remington, .22 Airgun
.24 6 mm 0.243 in .243 Winchester, 6 mm Remington, 6mm plastic airsoft BBs
.25 6.35 mm 0.25 in, 6.35 mm .25 ACP, 6.35x16mmSR a.k.a. .25 Auto and 6.35 mm Browning
.26 6.5 mm 0.264 in, 6.7 mm 6.5 x 55 mm, .260 Remington cartridges commonly known as 6.5 mm
.27 6.8 mm 0.277 in, 7.035 mm .270 Winchester, 6.8 SPC
.28 7 mm 0.284 in, 7.213 mm .280 Remington, 7 mm Remington Magnum, 7 x 57 mm, 7mm-08 Remington, 7 x 33 mm Sako commonly called 7 mm
.30 7.62 mm 0.308 in 30-06, .308 Winchester (7.62x51mm NATO), .300 Winchester Magnum, 7.82 Lazzeroni Patriot American ".30 caliber"
.303 7.62 mm 0.311 in - 0.312 in .303 British, 7.62x39, 7.62x54R, 7.62x25 Other ".30 caliber", note the mis-labelling of the Russian "7.62" rounds where neither the bore nor the groove diameter is 7.62mm
.323 8 mm 0.323 in 8x57mm IS, .325 WSM, 8 mm Remington Magnum, 8mm plastic (airsoft) BBs .32 caliber rifle cartridges
.357 9 mm 0.355-0.357 in .38 Special, .380 ACP, .357 Magnum, .357 SIG, .35 Remington, 9×19mm Parabellum, 9x18mm Makarov, .357 in certain new Crosman Pre-charged pneumatic (PCP) airguns. Handgun cartridges known as "38" are .357 caliber. Generally .357 for revolvers and rifles, .355 in autoloaders
.40 10 mm 0.400 in .40 S&W, 10 mm Auto
.50 12.7 mm 0.510 in (12.95 mm) .50 BMG, 12.7 x 108 mm M2 Browning machine gun and other heavy machine guns, long range rifles typified by Barrett Firearms Manufacturing products

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