The Mk 2 grenade (sometimes written Mk II) is a fragmentation type anti-personnel hand grenade introduced by the U.S. armed forces in 1918. It was the standard issue anti-personnel grenade used during World War II and in later conflicts, including the Vietnam War. Replacing the failed Mk I of 1917, it was standardized in 1920 as the Mk II, and redesignated the Mk 2 in 1945.
The Mk 2 was replaced by the M26-series (M26/M61/M57) and later M33 series (M33/M67). It was phased out gradually in service beginning with the Korean War. Due to the tremendous quantity manufactured during World War 2, it was in limited standard issue with the US Army and US Marine Corps throughout the 1950s and 1960s. The U.S. Navy was one of the last users when it was discontinued in 1969.
The Mk II was manufactured with grooves in the cast iron to enhance fragmentation and provide a better grip handling and throwing the grenade. These gave it the appearance of a pineapple and origin of that nickname. It was also commonly referred to as a "frag" grenade, in contrast to the Mk 3 concussion grenade.
The original Mk II grenade had a 3/8-inch threaded plug in its base covering the opening used to place the explosive filling. The improved "Mk IIA1" (a designation used informally by armorers, historians, and collectors but was never officially by the US military) introduced in 1942 was filled through the fuse well instead.
Low explosive Mk II grenades were filled with smokeless EC powder powder, which produced an adequate amount of fragmentation and did away with the need of a detonator. It was initially replaced by a small length of safety fuse terminated with a black powder igniter charge. Production grenades with the EC powder filler used the M10 series of igniting fuse.
High Explosive Mk II's used flaked or granular TNT. Pre-war Mk IIs with a TNT filler were identified with an all-yellow body as a warning to users. Wartime grenades were repainted olive drab for camouflage purposes with a narrow yellow band below the fuse. Repainted grenades usually lacked the yellow band.
The Mk II used the M5, M6, and M10 series fuses. These early fuses made a loud "bang" and produced sparks when activated. They had other problems as well. The M10-series' powder train made a "hissing" sound as it burned, potentially alerting the enemy of its presence. The M5 and M6 series sometimes prematurely detonated when the flash from the primer hit the TNT charge rather than the delay fuse. Moisture could get in under the foil fuse cap, causing the weapon to fail to detonate. Improved smokeless and (almost) silent fuses (like the M204-series) were later fitted after World War II.