While under experimentation by the Japanese as early as 1904-1905, the rifle grenade came into its own with the French during World War 1 (1914-1918). The rifle grenade broadened infantry-level firepower by allowing a standard service rifle to accept and fire a grenade projectile, adding range and lethality to the foot soldier's reach. Many rifle grenade designs soon emerged from various global parties and their use continued into World War 2 (1939-1945) and further. By the time of the Korean War (1950-1953), American forces had taken on the Belgian "ENERGA" series (detailed elsewhere on this site) but this was only intended as an interim move for an all-new indigenous rifle grenade design was already in the works - the M31 HEAT ("High-Explosive, Anti-Tank").
The M31 bettered the ENERGA by reducing internal complexity and overall weight while increasing battlefield performance and reliability. Design work began during the mid-to-latter part of the 1950s and this led to serial production beginning before the end of the decade. Its battlefield use primarily covered the anti-tank role but the grenade also held some value against infantry. Effective engagement ranges reached 115 meters while maximum ranges peaked at 185 meters from the shooter's position.
Overall weight was 709 grams with an overall length measuring 430mm. The projectile held a diameter of 66mm at its warhead. Its general shape was of a rocket and fins were used to stabilize the unit while in flight. Detonation was through a nose-actuated, base detonation arrangement relying in a piezoelectric fuze. The nose cap of the projectile held the piezoelectric crystal to which a lead wire was attached. This ran down to the fuze located at the base of the warhead. The forward section of the warhead was hollow so as to collapse upon impacting a surface. 254 grams of Composition B filling were used for detonation and engagement angles reached 65 degrees. A mechanical safety system was in play to prevent accidental detonation of the weapon during transport and handling.
The standard service rifle for American forces after the Korean War was still the classic M1 Garand of World War 2 fame so the M31 was designed with the Garand in mind and coupled with the M7A3 launcher. When the M14 semi-automatic rifle became the next American standard, the M31 was modified for use with this weapon system and brought along support for the M76 series launcher instead. In either case, the adapter was fitted directly to the muzzle of the rifle component and the grenade actuated through use of the M3 ballistic cartridge - these rounds specific to rifle grenade-launching in the American inventory. The projectile simply sat atop the launcher unit through its hollowed-out base.
The M31 only ever proved consistently effective against light armored vehicles and other "soft" targets as success was greatly reduced against the protection of a heavy tank. This led to the adoption of more potent man-portable systems such as the shoulder-fired M72 LAW (Light, Anti-Tank Weapon) which went on to replace the M31 in service with the American Army and Marine forces. The M31 continued to see service in the Vietnam War (1955-1975) but, by the end of the conflict, was given up for good. Many other global frontline forces moved in the direction of more potent, shoulder-fired weapons as well, leaving few today that still rely on some form of rifle-launched grenade projectile. The anti-infantry role of the rifle grenade was taken up by single- or multi-shot grenade launchers.