Caliber Conversions Table
By Matthew J. Boyd
The purpose of this conversion chart is to give you a rough idea of how the bullet diameters compare to each other. It is meant as a quick reference for when you're not sure how the different measured sizes relate. Or, for when you and your buddy are having a debate (9mm vs. .38 caliber, for instance).Just because a .30 cal bullet is the same nominal diameter as a 7.62mm bullet does NOT mean that you can interchange the bullets in firearms chambered in those sizes.
Cartridge length seriously comes into play when you start looking into making custom ammunition or switching similar bullets and brass around. You can NOT take a cartridge that is 2.5" long and expect it to cycle properly in a firearm that is chambered for a 2.4" long cartridge. If you try to do that, you will put yourself and others at risk. Or, you'll just look like a dummy when you have to have someone take apart your gun and un-jam it for you.
I made the mistake as a kid once and grabbed the wrong cartridge for a friend's .30-30. I did get the cartridge out safely, 3 hours later, after disassembling and reassembling the whole carbine.
Not all calibers accurately reflect the actual size of the bullet. Just like the marketing department to make sure that not all things are exactly as advertised. It's their job to bend the facts, take advantage of different types of measurements, and to confuse the average consumer while making company X more money in the process. Some calibers are named for the bore diameter, and some are named for the actual bullet diameter. Some are overestimated or rounded up to make them seem like a more powerful cartridge, or rounded down to have an even number. I'm sure, for example, that the famous .44 Magnum wouldn't have been so cool if it had been named the ".42 Magnum."
DO NOT try switching bullets around. Attempting to switch bullets of like calibers in different cartridges can be dangerous. Too large of a bullet can cause cycling problems, jams, or excessive pressure buildup. Too much pressure can cause a barrel to explode injuring the operator as well as nearby people. Too small of a bullet can cause cycling problems, or cause the bullet to rattle, rather than scream, out of the barrel. Make sure you have a micrometer to measure the true diameter of the actual bullet.
Formulae
1 inch = 1.00 Caliber = 25.4mm
Caliber x 25.4 = size in mm (Example: .45 caliber x 25.4 = 11.43 so .45 caliber = 11.43mm)
Metric designation / 25.4mm = size in Caliber (Example: 9mm / 25.4 = 0.35433 so 9mm = approximately .35 cal)
When dealing with calibers that have hyphens in them, discard the hyphen and the number after it. The number before the hyphen is the caliber of the bullet. The number after the hyphen often refers to how many grains of gunpowder was once used in the cartridge, or the year that the cartridge was made. (Example: .45-70 = .45 cal with 70 grains of black powder.)
Terminology
Brass - The cartridge case. This is usually made out of brass, hence the name. The case holds the powder inside, a primer in its bottom, and a bullet in its mouth. Assembled, these components constitute a cartridge.
Bullet - The projectile that is propelled downrange when a gun is fired. Bullet cores are usually made of lead that is often jacketed or semi-jacketed in copper or a copper alloy. Or not jacketed at all (a.k.a. cast or swaged lead bullets).
Bullet diameter - The actual diameter of a bullet, usually measured in decimal fractions of an inch (.308"). In Europe bullet diameters are typically expressed in millimeters (7.62mm). In recent years many cartridges have been named for their bullet diameter (.308 Winchester).
Bore diameter - The land to land diameter of the inside of a gun barrel, usually expressed in decimal fractions of an inch (.300). Traditionally cartridges have been named for the bore diameter of the guns that fire them (.270 Winchester).
Cartridge - The combination of the bullet, case, powder, and primer in a ready to fire configuration (the way you would buy them in a box from a store).
Case - The case is really a vessel that holds the powder inside, a primer in its bottom, and a bullet in its mouth. It is usually made of brass. Assembled, these components constitute a cartridge.
Firing pin - This is a small metal rod inside of a firearm that strikes the primer of a cartridge when the trigger is pulled and it is hit by the hammer. This causes the primer to ignite the powder inside the cartridge and propel the bullet out of the firearm. A long, heavy firing pin that is propelled by a stiff spring rather than being hit by a hammer is usually called a "striker."
Groove diameter - The groove to groove diameter of the inside of a gun barrel. This is usually the same as the diameter of the bullets that it is intended to fire.
Primer - This is a small cap that fits into the rear of the brass. This cap usually contains a highly sensitive explosive that is required to set of the powder inside of a cartridge.
Smokeless Powder - The nitro-cellulose based gunpowder that is used in most modern firearms. There are several manufacturers of smokeless powder and numerous individual types of smokeless powder. Among the best known brands in North America are Alliant, Hodgdon, IMR, Norma, and Winchester/Olin.
Striker - A long, relatively heavy firing pin that is propelled by a stiff spring rather than being hit by a hammer.
Common name / Bore diameter (inches) / Bullet/Groove diameter (inches) / Bullet/Groove
diameter (MM)
.50 cal / .500" / .510 / 12.954
.475 cal / - / .475 / 12.065
.45 cal rifle / .450 / .458 / 11.633
.45 revolver / - / .454 / 11.531
.45 revolver / - / .452 / 11.480
.45 ACP / - / .451 / 11.455
.44 Mag. / - / .429 / 10.896
.416 cal / - / .416 / 10.566
.41 cal / - / .410 / 10.414
.40cal/10mm / - / .400 / 10.160
.375 cal / - / .375 / 9.525
9.3mm / - / .366 / 9.300
.35 cal rifle / .350 / .358 / 9.093
.357 Mag
.38 Spec / .350 / .357 / 9.067
9mm, .380
.357 SIG / - / .355 / 9.017
.348 cal / .340 / .348 / 8.839
.338 cal / .330 / .338 / 8.585
.33 cal / - / .333 / 8.382
8mm / .315 / .323 / 8.204
.32 Special / .313 / .321 / 8.153
.32-20 cal / - / .312 / 7.924
.303 cal
7.65mm / .303 / .312 / 7.924
7.62x39 / .300-.303 / .311 / 7.899
.30/.300 cal / .300 / .308 / 7.832
7.5mm / - / .295 / 7.500
.28 cal/7mm / .275-.276 / .284 / 7.213
.270 cal / .270 / .277 / 7.035
.26cal/6.5mm / .256 / .264 / 6.705
.25 cal / .250 / .257 / 6.527
.243/6mm / .236 / .243 / 6.172
.22 cal / .218/.219 / .224 / 5.689
.20 cal / - / .204 / 5.080
.17 cal / - / .172 / 4.368
BB / - / .177-.180 / 4.495