While the U.S. Army approved and adopted an anti-personnel rifle grenade in the "M17" of 1941, it also took on stocks of a High-Explosive, Ant-Tank (HEAT) rifle grenade as the "M9". This weapon was the standardized HEAT rifle grenade for American forces in World War 2 (1939-1945) and appeared in similar design to other rifle grenade types of the period - actuation through use of a blank rifle cartridge and the grenade seated over the muzzle of a standard service rifle - namely the M1 Garand. The operator could engage targets or target areas through direct line-of-sight or indirect line-of-sight fire.
The M9 featured a body of steel sheeting and its warhead was of a shaped-charge design suitable for defeating armor up to 2-inches in thickness (though some factors such as angle came into play). The M9 was detonated through a fuse located in its base. The overall shape of the grenade was very torpedo-like with a smooth, rounded head, tapered tail section and finned aft end. The fins provided stability during the grenade's flight path. The complete grenade system added some 1.3lbs to the weight of the service rifle while also increasing its length. The M9A1 was an improved version of the M9 and supplanted the latter in frontline service by the end of the war in 1945.
The M9 gave good service throughout its time in the war. The grenades could also be used beyond its original anti-armor role and engage fortified structures. The M11 series were practice versions of the M9 that could be reused for training. These were produced across marks M11A1, M11A2, M11A3, and M11A4 - each successive design taking into account improved mass-production practices as the war evolved.