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Commonly known as the 'Thumper' or 'Blooper', this weapon first appeared during the Vietnam war and closely resembled a large bore, single barrel, sawn-off shotgun. The first M79 Grenade launchers were delivered to the US Army in 1961.

The M79 was designed as a close support weapon for the infantry, with two weapons being issued to each rifle squad. The tactical use of the weapon required the gunner (grenadier) to be dedicated to the weapon and only carried a pistol as a side arm. the M79 was intended to bridge the gap between the maximum throwing distance of a hand grenade, and the lowest range of supporting mortar fire - between 50 and 300 meters - and thus gave the squad an integral indirect fire weapon. With a length of 737mm (barrel = 355mm) and a loaded weight of 3kg, (6 and a half pounds) the M79 was an ideal weapon in the close terrain of Vietnam.

The M79 was a single shot, shoulder fired, break-barrel weapon which fired a spherical 40mm diameter grenade loaded directly into the breech. It had a rubber pad fitted to the shoulder stock to absorb some of the shock. The M-406 40mm HE grenades fired from the M79 traveled at a muzzle velocity of 75 meters per second, and contained enough explosive within a steel casing that upon impact with the target would produce over 300 fragments at 1,524 meters per second within a lethal radius of up to 5 meters. Stabilized in flight by the spin imparted on it by the rifled barrel the grenade rotated at 3,700rpm, this in turn, after approximately 15 meters of flight, armed the grenade.

For close range fighting the Army came up with two types of M79 rounds. The first was a flechette round ( or Bee Hives round) which housed approx 45 small darts in a plastic casing, these rounds were issued on an experimental basis. Later this round was replaced by the M-576 buckshot round. This round contained twenty-seven 00 buckshot which on firing was carried down the barrel in a 40mm plastic sabot which slowed down in flight so that the pellets could travel in their forward direction un-aided. The M79 could also fire smoke grenades (both standard and parachute), CS gas, and flares.

The M79 had a large flip up sight situated half way down the barrel, with a basic leaf foresight fixed at the end of the barrel. The rear sight was calibrated up to 375 meters (410 yds) in 25 meter (27.3 yds) intervals. In the hands of a good experienced Grenadier the M79 was highly accurate up to 200 meters. Later in the war the M79 was superseded by the M203.