Common Ammunition Conversion Guide

Here is a prepper's guide to ammunition calibers and their imperial or metric sizing.

 

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  
.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 300 AAC Blackout, 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 .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 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  
.45 11.43 mm 0.450 in .45 ACP, .45 GAP, .454 Casull, .45 Long Colt  
.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