To continue more about caliber and gauge.

 

The 12 gauge shotgun bore is 0.729 inches.  It takes 12 lead balls to make one pound; therefore, the British called this a 12 bore or 12 gauge.  Other shotguns such as the 10 gauge, 16 gauge, 20 gauge, and 28 gauge have different bore diameters and require these numbers of lead balls to make one pound.  The difference is the .410 shotgun shell.  The bore is expressed in inches -- and I have now idea why.

 

 

Caliber can mean two different things: (1) the diameter of the bore in millimeters/inches or (2) the length of the barrel in millimeters/inches.

 

Cartridges are classed under the metric system that specifices bore in millimeters, case length in millimeters, and rim type (if required).  The rim type is specified by a letter: (1) none - rimless, (2) B - belted, R - rimmed, RB - rebated rimless.  Example: the 20x110RB anti-aircraft round used by the World War 2 Oerlikon anti-aircraft gun.

 

U.S. cartridges are more different to categorize because they are a composite of black/smokeless powder designations as well as commercial/military designations.

 

Let's look at sample metric and U.S. designations:

 

7,62x54R (Russian rimmed rifle round adopted in 1891).

7.62 Russian (U.S. designation for same)

 

7,62x39 (Russian assault rifle cartridge)

7.62 Russian Short (commonly identifed by U.S. manufacturers by its metric equivalent)

 

7,62x25 (Russian pistol round for the Tokarev pistol, also for C96 Mauser pistol)

7.63 Mauser (U.S. designation for same)

 

5,56x45 NATO (U.S. assault rifle cartridge)

.223 Remington (commercial equivalent)

 

7,62x63 (metric designation for .30-06)

.30-06 Springfield (commercial) or cal. .30 (government)

 

7,62x51 NATO (metric designation)

.308 Winchester (commercial equivalent; brass is softer than G.I. and not suitable for roller-locked rifles like the CETME and HK91/G3 series)

 

While the metric system may seem simple, it is not.  For instance the 7,92x57 Mauser cartridge was loaded with two different diameter bullets, a .318 diameter and the more common .323.

 

As far as U.S. designations go, you'd better know your rifle and know what it's supposed to shoot.

 

The .45-70-405 is used in the M1873 Springfield rifle (.45 caliber, 70 grains of black powder, 405 grain bullet).  The M1873 Springfield carbine uses a .45-55-405 loading.

 

The .30-40 Krag (called .30-40-220) used 40 grains of smokeless powder with a 220 grain bullet.  Bullet weight was dropped from the designation and it was also known as the .30 Government.  Later loadings reduced the weight of the bullet to 173 grains.

 

The .30-06 was a smokeless power round developed for the U.S. military in 1903 with a round nose bullet.  Three years later it was improved with a more aerodynamic 173 grain bullet called the caliber .30 M1.  A later loading reduced the weight of the bullet to 150 grains and this round was called the caliber .30 M2.  Commercial loadings call this round the .30-06 Springfield (bore diameter, year of adoption, place of adoption).  This is probably the most flexible rifle round ever devised.

 

The 7,62x51 NATO was adopted in 1957 when the U.S. Army changed to the metric system (NATO standardization).  The .308 Winchester (bore diamenter, manufacturer) make a commercial equivalent.  However, there are minor dimensional differences.  These differences only come into play if a .308 Win round is used in some military rifles or machine guns.  Military brass is stronger and has harder primers; commercial brass is softer and so are the primers.  This can result in slam fires in some firearms or split cases.  Delayed blowback, roller-locked rifles and machine guns are extremely hard on military brass.  Examples: Spanish CETME rifle, German HK91/G3 rifle, French AAT-52 general purpose machine gun (GPMG).

 

The 5,56x45 NATO was adopted about 1964 for the Colt AR-15, later M16 assault rifle.  The round was developed from the .223 Remington cartridge and are interchangeable.

 

 

Barrel length is measured in calibers for guns above 20mm (0.79 inch).  To find the length of the barrel multiply the bore diameter in millimeters or inches and multiply by the length of the gun tube in calibers. 

 

Examples:

World War 2 Oerlikon 20/L70 AA gun -- 20 x 70 = 1,400mm (55.1 inches)

World War 2 5"/38 dual purpose gun -- 5 (127) x 38 = 190 inches (4,826mm)

World War 2 16"/50 naval rifle -- 16 (406) x 50 = 800 inches or 66 feet (20,320 mm)