Measuring Up: Gauge vs. Caliber

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)

We teach some of this stuff about shotgun gauges and calibers (not the heavy ordnance part) in the curricula for our Hunter Safety class.

 

This is anecdotal evidence, but the longer I work with the younger shooters, the more I discover all the stuff they just don't know. 

 

One of our major concerns is putting the right ammo in the right firearm.  We also caution our HS trainees about the classic 20 gauge/12 gauge blow-up -- drop in a 20 gauge and then follow it with a 12 gauge and fire.  The barrel blows up just about where your forearm, wrist and hand are.  Another is why you need to load softnose lead or bluntnose hollow points in tube feed rifles and not pointed (or "spitzer") bullets.  The recoil acts on the rounds and drives the leading round's primer into a pointed bullet where it acts as a firing pin.  The result is a magazine explosion and you go to the emergency room.  We caution them about barrel obstructions -- like shooting a sabot slug in a shotgun barrel with a choke tube installed or getting mud/snow in the barrel or have something like stuck in the barrel -- and how the barrel can blowup or the gun.  Either condition can seriously hurt or kill you.

 

Most shooters depend on the caliber or gauge being stamped on the barrel of the firearm to tell them what kind of ammo to use.  Modern firearms do, but older ones don't and neither do ex-military firearms.

 

Example: ex-military .45 pistols come in both black powder and smokeless powder loadings: .45 S&W Schofield (for their top break M1875 revolvers); .45 Long Colt for the M1873 Colt Single Action Army; .45 ACP for the M1911 series self-loaders and M1917 series revolvers.  There are major non-interchangeability issues here.

 

Example: ex-military .30 rifles come in .30-40 Government for M1892-M1898 Krag-Jorgensen rifles (a rimmed case); .30-06 Springfield or cal. .30 M1 or M2 for M1903, M1917, and M1 rifles; .30 Carbine for M1, M1A1, M2, and M3 carbines; 7,62x51 NATO (.308 Winchester) for M14 rifles.  There are major non-interchangeability issues here.

 

The bottom line is this: what you don't know CAN hurt or kill you.  I'm glad to add to the knowledge pool.