I often have blog readers and consulting clients send me questions about firearms calibers. They are often confused, but this not always their fault. It is a confusing, complex, and often arcane topic. A lot of the facts that you will find are mere trivia, but since safety is an issue–(we mustn’t fire the incorrect ammo in our guns!)–it is important to study.
To begin: Metric caliber designations are usually written with an “x” in the middle to distinguish the bore diameter and case length. For example, the 6.5×55 designation tells us that the cartridge uses a 6.5 mm bullet, and a case that is 55 mm in length.
The following table is partly Creative Commons licensed (courtesy of Wikipedia), with additions by JWR (such as 7.5mm, 7.7mm, .375 H&H, .455 and .577) and minor corrections and comments. Hence, I am retaining my moral rights.
|
Bore (Inches) |
Metric Equiv. |
Bullet Dia. (Inches) |
Typical Cartridges | JWR’s Comments |
| .172 | 4.4mm | .172 |
.17 HMR, |
Varmint cartridges |
| .177 | 4.5mm | .177 Pellet, .175 BB | Airgun .177 caliber | Common “BB Gun” and “Pellet Gun” ammo |
| .204 | 5mm | .204 | .204 Ruger, 5 mm Remington Rimfire Magnum | Dubious market longevity, so beware! |
| .220 | 5.45mm | .220 | 5.45 x39mm | AK-74 series. (.220 Swift is actually a .223) |
| .22 |
5.56mm 5.7mm |
.223 | .222 Rem., .223 Rem., .220 Swift, FN 5.7×28, .22-250, etc. | A bullet diameter of .224 is used with some cartridges |
| .228 | 5.8mm | .228 | .228 Ackley Magnum | |
| .243 | 6mm | .243 | .243 Winchester, 6mm Remington | Actually, it is 6.2mm |
| .25 | 6.35mm | .25 | .25 ACP | |
| .257 | 6.5mm | .257 |
.256 Win. Magnum, .25-06, .257 Roberts, |
|
| 6.5mm | .264 | 6.5×55 Swedish | Perfect for deer-sized game | |
| .264 | 6.7mm | .264 | .264 Win. Magnum | |
| .270 | 6.8mm | .270 | .270 Winchester, .270 Weatherby Mag. | Great for antelope |
| .280 | “7mm” | .284 | .280 Ross, 7×57 Mauser, 7mm Magnum, .280 Remington | Actually, it is a 7.2mm bullet, but called 7mm. |
| 7.5mm | .307 | 7.5×55 Swiss | Schmidt-Rubin | |
| .30 US | 7.62mm | .308 | .30-06, .308 Winchester | Ubiquitous! |
| .30 Euro | 7.62mm | .311 | 7.62×39, .303 British | 7.62×39 is the world’s most common centerfire military cartridge. |
| .30 Mauser | 7.63mm | .311 | .30 Mauser | Broomhandle Mauser. |
| .32 | 7.63mm | .312 | .32 ACP, .32 S&W | |
| 7.7mm | .311 | 7.7 x58 Jap | WWII Arisaka | |
| .323 | 8mm | .323 | 8×57 Mauser .325 WSM, 8mm Remington Magnum, | 98 Mauser, et al. |
| .338 | 8.58mm | .338 | .338 Lapua, .338 Winchester Magnum, .338 Federal | Becoming popular for counter-sniper rifles |
| .348 | 8.75mm | .348 | .348 Winchester | |
| .355 | 9mm | .355 | 9mm Parabellum, .380 ACP | |
| .357 (“.38”) | 9mm |
.357 – .359 |
.38 S&W, .357 Magnum | A “.38 S&W” is NOT 0.38″! |
| 9.22mm | .363 | 9mm Makarov | The Russians just have to be a little different. | |
| .374 | 9.3mm | .374 | 9.3×62 Mauser, 9.3×72 R | |
| .375 | 9.5mm | .375 | .375 H&H Magnum | The world’s most popular dangerous game cartridge |
| .400 | 10mm | .400 | .38-40, .40 S&W, 10 mm Auto | A .38-40 is actually a .400 |
| .41 | 10.25mm | .410 | .41 Magnum | Sadly, fading away |
| “.405” | 10.25mm | .411 | .405 Winchester | Misnamed, but a great cartridge |
| .408 | 10.4mm | .408 | .408 Chey Tac | Based on the venerable .505 Gibbs case |
| .404 | 10.75mm | .423 | .404 Jeffery | |
| .41 Swiss | 10.4mm |
~.430 +/- |
.41 Swiss / Vetterli | The famous Vetterli cartridge |
| .416 | 10.6mm | .416 | .416 Barrett, .416 Rigby | |
| .43 | 11mm | .430 | .43 Spanish | Remington Rolling Block |
| .44 | 10.8mm |
.427- .430 |
.44 Special, .44 Magnum | |
| .45 | 11.45mm |
.451 .452 |
.45 ACP, .45 Colt, etc. | |
| .454 | 11.53mm | .454 | .454 Casull | Bear Medicine |
| .455 | 11.53mm | .454 | .455 Eley | British revolvers, often reworked to fire .45 ACP. |
| “.476” | 11.53mm | .454 | .476 Enfield | aka “.455/476”. Named for its neck diameter |
| “.450” | 11.6mm | .455 | .450 Adams | Yes, a “.450 Adams” had a bigger bullet than a “.455” |
| .458 | 11.6mm | .458 | .458 Winchester Magnum, .45-70 | Big game rifle |
| .460 | 11.6mm | .458 | .460 Weatherby | Actually a .458, but called a .460 for marketing |
| .475 | 12mm | .475 | .475 Linebaugh | |
| .480 | 12mm | .475 | .480 Ruger | Actually .475″ bore, but .480 sounds better |
| .500 | 12.7mm | .500 | .500 S&W Magnum | Polar Bear Medicine |
| .50 | 12.7mm | .510 | .50 AE, .500 S&W, .50 Beowulf, .50 BMG, 12.7 x 108 mm, etc. | More correctly, they are .51 caliber. |
| .505 | 12.8mm | .505 | .505 Gibbs | African big game rifle |
| .577 | 14.5mm | .570 | .577 Snider | British service rifle and carbine |
| “14.5mm” | 14.88mm | .586 | 14.5x114mm (PTRS-41) | Light cannon, but there are now “sniper rifles” chambered in 14.5mm [1] |
| .68 | 17.27mm |
.675- .695 |
Nelspot Paintballs | Paintball Markers [2] |
| 20mm | .787 | 20×102mm, etc. | Light cannon | |
| .950 | 24.13mm | .950 | .950 JDJ [3] | Based on the 20 x 102 mm Vulcan case |
| 30mm | 1.18 | 30 x 113 mm [4], etc. | Light cannon |
As you can see, there are some amazing inconsistencies in cartridge designations. (See, for example, .405 Winchester and .38-40.) As Jim Keenan at the Firing Line Forums aptly [5] put it: “It is usually best to just accept cartridge names; trying to figure out reasons for the names leads to insanity.”)
One regular source of confusion in cartridge naming is whether “bore diameter” refers to the rifling land dimension versus rifling groove dimension [6]. For example, most .303 British rifles have a land-to-land diameter of .303 and a groove-to-groove diameter of .311. So when you handload a “.303” cartridge, you actually use a .311 diameter bullet.
Specifications have also changed for a few types of ammunition. One example is the 8mm Mauser cartridge. Early production 8×57 rifles used 8.08 mm (.318 caliber) bullets, but the later guns used 8.2 mm (.323 caliber) bullets.
Now, on to the realm of shotguns, where life is simpler but there is still some minor confusion.
Common Production Shotgun Bores and Gauges in North America
| Designation |
Bore |
Bore Diameter, Metric |
Round Balls Per pound | JWR’s Comments |
| .410 | .410 | 10.41mm | 67.62 | Not truly a “Gauge”. (If it were, it would properly be a “67 Gauge”) |
| 28 Gauge | .550 | 13.97mm | 28 | Uncommon gauge, loved by some quail and dove hunters |
| 20 Gauge | .615 | 15.63mm | 20 | Second most common gauge in the U.S. and Canada |
| 16 Gauge | .663 | 16.83mm | 16 | Fading in popularity |
| 12 Gauge | .729 | 18.53mm | 12 | The most common gauge in the U.S. and Canada |
| 10 Gauge | .775 | 19.69mm | 10 | Revived popularity, due to the mandate of lead-free shot for waterfowl hunting in the U.S. |
Note that there are presently enormous revolvers being marketed that can fire both .45 Colt cartridges and .410 shotshells. (The Taurus “Judge” series.) I attribute the popularity of these revolvers to: A.) Ignorance of ballistics, and B). The unerring willingness of the American people to spend their money on impractical toys.