Copyright International Ammunition Association, 2007. All rights
reserved.
Cartridge of the Month
October 2007
40mm Grenade, M406 High Explosive
Specimen and photos courtesy of CRITTENDEN SCHMITT ARCHIVES
One of the first rounds standardized for use with the shoulder fired M79 Grenade Launcher in the Vietnam era, this round was later used with the M203 launcher mounted under the M16 rifles. These are "low velocity" grenades, different from later "high velocity" grenades and launchers which are also 40mm.
This weapons system uses many innovative ideas to achieve a lethal weapon with light weight.
The Launchers used aluminum barrels for light weight which necessitated low chamber pressures. To achieve these pressures an minimizing recoil, the cartridge uses a dual "high pressure-low pressure" chamber design.
When the firing pin strikes the primer, the flash ignites the propellant in the brass powder-charge cup inside the high-pressure chamber. The burning propellant produces 35,000 psi chamber pressure, which ruptures the brass powder-charge cup at the vent holes. This allows the gases to escape to the low-pressure chamber in the cartridge case, where the pressure drops to 3,000 psi and propels the grenade from the muzzle at a velocity of 250 feet per second.
The rifling in the barrel causes the grenade to rotate. The grenade's 37,000-rpm right-hand spin stabilizes the grenade during flight and applies enough rotational force to arm the fuze. The elements of the fuze can be seen within the nose cover. The impact fuze is behind this, and the cylindrical white substance is the main charge.
The casualty causing fragments come from the spherical steel body with the pre-fragmented pattern.
This round has a maximum range of 400 meters, but a maximum effective range of about 150 meters at a point target or 350 meters for an area target.