Understanding Metric and Inch Bore Dimensions

Friday, Jun 3, 2011

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 6.5x55 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,
.17 Remington

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 5.7x28, .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 6.5x55 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, 7x57 Mauser, 7mm Magnum, .280 Remington Actually, it is a 7.2mm bullet, but called 7mm.
  7.5mm .307 7.5x55 Swiss Schmidt-Rubin
.30 US 7.62mm .308 .30-06, .308 Winchester Ubiquitous!
.30 Euro 7.62mm .311 7.62x39, .303 British 7.62x39 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 8x57 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 9.3x62 Mauser, 9.3x72 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
.68 17.27mm

.675-

.695

Nelspot Paintballs Paintball Markers
  20mm .787 20×102mm, etc. Light cannon
.950 24.13mm .950 .950 JDJ Based on the 20 x 102 mm Vulcan case
  30mm 1.18 30 x 113 mm, 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 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. 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 8x57 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
Diameter, Inches

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.


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