by Frank Moss

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

Inch Caliber

Metric Caliber

Typical Bullet Diameter

Common Cartridges


.20, .2045 mm0.204 in.204 Ruger, 5 mm Remington Rimfire Magnum
.225.56 mm0.223-0.224 in.22 Long Rifle, .222 Remington, .223 Remington, 5.56×45mm NATO, 5.7 x 28 mm, .22-250 Remington, .22 Airgun
.246 mm0.243 in.243 Winchester, 6 mm Remington
.256.35 mm0.25 in, 6.35 mm.25 ACP, 6.35x16mmSRa.k.a. .25 Auto and 6.35 mm Browning
.266.5 mm0.264 in, 6.7 mm6.5 x 55 mm, .260 Remingtoncartridges commonly known as 6.5 mm
.276.8 mm0.277 in, 7.035 mm.270 Winchester, 6.8 SPC
.287 mm0.284 in, 7.213 mm.280 Remington, 7 mm Remington Magnum, 7 x 57 mm, 7mm-08 Remington, 7 x 33 mm Sakocommonly called 7 mm
.307.62 mm0.308 in300 AAC Blackout, 30-06, .308 Winchester (7.62x51mm NATO), .300 Winchester Magnum, 7.82 Lazzeroni PatriotAmerican “.30 caliber”
.3037.62 mm0.311 in – 0.312 in.303 British, 7.62×39, 7.62x54R, 7.62×25Other “.30 caliber”, note the mis-labelling of the Russian “7.62” rounds where neither the bore nor the groove diameter is 7.62mm
.3238 mm0.323 in8x57mm IS, .325 WSM, 8 mm Remington Magnum.32 caliber rifle cartridges
.3579 mm0.355-0.357 in.38 Special, .380 ACP, .357 Magnum, .357 SIG, .35 Remington, 9×19mm Parabellum, 9x18mm MakarovHandgun cartridges known as “38” are .357 caliber. Generally .357 for revolvers and rifles, .355 in autoloaders
.4010 mm0.400 in.40 S&W, 10 mm Auto
.4511.43 mm0.450 in.45 ACP, .45 GAP, .454 Casull, .45 Long Colt
.5012.7 mm0.510 in (12.95 mm).50 BMG, 12.7 x 108 mmM2 Browning machine gun and other heavy machine guns, long range rifles typified by Barrett Firearms Manufacturing products



By Frank Moss

Well traveled in Europe and North Africa, Frank Moss has lived in England, Germany and Italy thanks to sabbaticals taken by his father, an award winning physicist. Notable positions held during his almost 20 year IT career are founding engineer of IBM's SmartCloud, Cloud Architect for SPAWAR, and Cloud Architect for CenturyLink's FedRAMP Cloud Project. He specializes in architectural security compliance (secure by design) and holds a C|EH. When not working, Frank enjoys outdoor activities, shooting, camping and spending time with his wife and children. You can find Frank at one of his two sites: or

8 thoughts on “Common Ammunition Conversion Guide”
  1. The .45 Long Colt is a moniker dreamed up by gun scribes. In Sam Colt’s writings, he always referred to that cartridge as the 45 Colt.. It’s one of the few cartridges that made the transition from Black Powder Rim fire to Smokeless powder center fire.

    1. It was actually coined by the US Army because after adopting the .45 Colt, they also adopted the .45 Schofield. It used a shorter cartridge so although the Colt SAA could use the Schofield cartridges, it didn’t work the other way around. When the Army shipped boxes marked ‘Cartridge, .45 Caliber’ units with Schofields were getting .45 Colt which they couldn’t use. To address the problem, the Army started marking the .45 cartridges for the Colt as .45 Long Colt so they wouldn’t get sent to units with the Schofield. After the Schofield went out of service, the name stuck.

  2. To Ron,

    According to Frank Moss’ chart in his article on Conversion Guide (see above),
    .243 = 6 mm

  3. Can anyone please explain when Glock created this-1 (Glock 179mm handgun) with 2 31-round magazines and a 17-round magazine? How could you tow this into the lobby of a county courthouse? Or, conceal it in a duffel bag with all that ammo &, &, & all the other weapons? How do you step up to someone who can even fire this? Yep, MSM strikes again with one of its smear campaigns against someone who make a stupid reaction to a stupid law. So they need to convict him in the public view before he has even had his initial hearing.

    I am not defending what he did. Just informing everyone to be aware that this is what ‘you’ could face. How you handle it is up to you. But be aware that the MSM will start a campaign against you weather you are right/wrong or indifferent.

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