The M31 HEAT is a fin-stabilized anti-tank

rifle grenade A rifle grenade is a grenade that uses a rifle-based launcher to permit a longer effective range than would be possible if the grenade were thrown by hand. The practice of projecting grenades with rifle-mounted launchers was first widely used du ...

designed in the late 1950s to replace the Belgian

ENERGA Energa SA is a Polish corporate group which deals in the generation, distribution, and supplies electricity to approximately 2.7 million people in Northern Poland. Energa is Poland's third largest distribution network operator serving North and C ...

rifle grenade which was adopted by the US Army and US Marines as an emergency stop-gap measure during the Korean War. Like the ENERGA, it has a nose-initiated, based-detonated

HEAT In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...

warhead, but unlike the ENERGA, the mechanical impact fuse system is replaced with a less complex and more reliable

piezo-electric Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word ''p ...

fuse system which also allows higher angles of impact, up to 65 degrees.

History

The M31 was originally designed to be fired only from the

M1 Garand The M1 Garand or M1 rifleOfficially designated as U.S. rifle, caliber .30, M1, later simply called Rifle, Caliber .30, M1, also called US Rifle, Cal. .30, M1 is a semi-automatic rifle that was the service rifle of the U.S Army during World War ...

, but could also be fired from both the M14 and

M16 The M16 rifle (officially designated Rifle, Caliber 5.56 mm, M16) is a family of military rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-roun ...

rifles.Article title
, Limited Range Test of the M16 Rifle with Eight Types of Rifle and Hand Grenades. By Dewey Calfee. January 1965. Directorate of Armament Development 4, Research and Technology Division, Air Force Systems Command, Eglin Air Force Base, Florida To launch the M31, a detachable spigot-type grenade launcher (M7A3 launcher for M1 rifle, and M76 launcher for the M14 rifle) is fitted to the muzzle of the rifle. A M3 ballistic cartridge (two are supplied in the packing container with each grenade and are crimped to indicate the cartridges are only for launching rifle grenades) is loaded into the rifle's chamber. The hollow tail unit of the rifle grenade is fitted over the grenade launcher. Official military manuals recommend that the M31 HEAT be fired from either the standing or kneeling position and that it is only accurate against armored vehicles if fired at extremely close ranges. While claimed to be effective against main battle tanks (except in frontal engagements) and lightly armored vehicles when first introduced, in 1972 the US Army stated in its revised anti-armor warfare manual that the M31 HEAT was only effective against light tanks and thin-skinned vehicles.

Design

On impact, the nose cover collapses, crushing a crystal-like material, which sends an electric current through a separate wire to the warhead's detonator, located in the base of the warhead. A mechanical safety, comprising a set back system located in the warhead's base, grounds the firing circuit and prevents the accidental explosion of the warhead. On firing, the sudden launch acceleration causes the set-back's three disks to rotate 90 degrees, each in succession to the other, with the rotation of the third disk removing the grounding after approximately 10 meters of flight and completing a firing circuit for the current to flow from the nose to the detonator in the base. Compared to the ENERGA, the M31 is slightly lighter in weight and has a smaller-diameter warhead—i.e. 75mm vs 66mm. Penetration for the M31 is estimated to be ''200 mm / 8 inches'' for steel armor plating and twice that estimate for concrete. The warhead technology developed for the M31 was used for the future

M72 LAW The M72 LAW (light anti-tank weapon, also referred to as the light anti-armor weapon or LAW as well as LAWS: light anti-armor weapons system) is a portable one-shot unguided anti-tank weapon. The solid rocket propulsion unit was developed in th ...

antitank rocket.

Status

Various US military manuals issued in 1972 still had sections on the M31, but by the end of the Vietnam War, both the US Army and US Marines had essentially phased out muzzle-launched rifle grenades, in favor of the

disposable rocket in the anti-armor role and the

M203 The M203 is a single-shot 40 mm under-barrel grenade launcher designed to attach to a rifle. It uses the same rounds as the older stand-alone M79 break-action grenade launcher, which utilizes the high-low propulsion system to keep recoil forces ...

under-barrel grenade launcher in the squad fire-support role. In the 1977 revision of US military anti-armor warfare manuals, the M31 HEAT was no longer listed.One of the reasons was that, with the phase-out of the heavier 7.62mm assault rifles for the lighter 5.56mm assault rifles, they no longer had an infantry weapon that could launch a heavy antitank rifle grenade
The 2001 U.S. Army TM 43-0001-29 C2, "Data Sheets for Grenades" details the Grenade, Rifle, HEAT M31 on pages 3-3 and 3-4 with DODIC ordering codes.

References and notes

* FM 23-3 Tactics, Techniques and Concept of Antiarmor Warfare, published US Army August 1972 * Jane's Infantry Weapon's System 1976, page 452, Watts Publishing
US Rifle Grenades, WW2 And After - inert-ord.net
{{commons category, M31 HEAT Anti-tank grenades Infantry weapons of the Cold War Rifle grenades Grenades of the United States