Ordnance Notes -- by Bob Stoner GMCM (SW) Ret.

Ordnance Terms Glossary

As I have been revising these Ordnance Notes, it stuck me that there may be readers who are unfamiliar with some of the terminology and expressions that I have used while writing them. As a result, I have put together this glossary to help the reader understand some of the technicalities used throughout them. I have not tried to arrange things in any particular order, but as I ran across items in the course of my revision activities. This is very much a work in progress, so come back frequently to check for additions.

CALIBER. This term has distinct meaning that depend upon whether we are talking about small arms (below 20mm [.79]) or major-sized guns (above 20mm).

In small arms, the term usually refers to the size of the bore in hundredths of an inch or millimeters. In the inch system, the bore diameter is expressed in inches (military) or it may be in inches with additional modifiers (commercial). For example:

Commercial Cartridge (U.S.) Military Cartridge (U.S.)

            .45 Long Colt .45 Government
.45 Auto .45 or .45 Automatic Colt Pistol (ACP)
9mm Luger or 9mm Parabellum 9mm NATO
.45-70 Government .45-70-505, .45 Springfield
.30-40 Krag .30 Government, .30 Krag
.30-06 Springfield .30 Springfield, .30-06
.308 Winchester 7.62mm NATO
.223 Remington 5.56mm NATO

For metric calibers, both bore diameter and case length in millimeters are specified, followed by a letter that indicates whether the cartridge case is rimmed (R), semi-rimmed (SR), or rimless (none). For example:

U.S. Cartridge Designation Metric Cartridge Designation

             9mm Luger or 9mm Parabellum 9x19mm NATO
.30-06 Springfield 7.62x63mm
.308 Winchester 7.62x51mm NATO
.223 Remington 5.56x45mm NATO
9mm Makarov 9.2x18mm
.30 Mauser 7.62x25mm
.30 Russian 7.62x54mmR
.30 Russian Short 7.62x39mm
.50 Browning 12.7x99mm NATO
40mm Grenade 40x53mmSR Grenade

For large caliber guns designations are by bore diameter and the length of the barrel in calibers. For example, a 3-inch/50 gun has a bore diameter of 3 inches and a barrel length of 150 inches (3 x 50 = 150 inches) or 12.5 feet. Metric guns are similar but use the letter L before the barrel length. For example, a 20mm L70 gun has a bore diameter of 20mm [.79-inch] and a length of 1400mm [55.12 inches].

Army and Air Force Ordnance Designations.

  • Type classified as standard: M for model, as in M16.

  • Type classified as alteration: A for alteration (plus a number), as in M16A1.

  • Classified as experimental: X for experimental, as in XM21.

  • Classified as experimental modification: E for experimental modification (plus a number), in XM177E2.

Navy Ordnance Designations.

  • Type classified as standard: Mk for mark, as in Mk 16.

  • Type classified as modification: Mod for modification (plus a number), as in Mk 16 Mod 5.

  • Classified as experimental: EX for experimental, as in EX29.

  • Classified as experimental modification: dash (plus a number), as in EX29-1.

Cyclic Rate. The theoretical maximum rate of fire of an automatic weapon.

Maximum Rate of Fire. The number of rounds per minute that the gunner of an automatic weapon can fire including magazine or belt changes.

Sustained Rate of Fire. The number of rounds per minute that the gunner can fire without overheating the weapon.

Casualty. An ordnance system that fails to perform as intended. It may be due to faulty ammunition, broken parts, an electrical problem, or some other cause.

Malfunction. An interruption in the cycle of operation of an ordnance system usually caused by ammunition, but may have another cause. It can be used interchangeably with Casualty.

Hang Fire. When the firing pin of the weapon hits the primer, the primer fails to go off. No hang fire has ever lasted more than 30 seconds. If the weapon fails to fire after 30 seconds, it is classed as a Misfire.

Misfire. If a weapon has not fired after 30 seconds, it is a Misfire. Each and every ordnance system has a designated misfire procedure that must be followed to remove the live ammunition from the gun or launcher. This information is in Ordnance Pamphlet 4 – Clearing Live Ammunition from Guns.

Cook-off. The heat generated from the firing of automatic weapons or large guns causes the barrel to become hot to the point where the heat radiated by the barrel may cause a misfire to go off. That is why the weapon is pointed in a safe direction when misfire procedures are in progress. For weapons with explosive projectiles, it is critical that if a cook off occurs, it is the propellant and not the explosive filler. OP4 contains tables that tell ordnance personnel what the cook-off time limits are for explosive projectiles in different caliber guns.