"BLOOPER" THE M-79 GRENADE LAUNCHER:

The M79 was designed as a close support weapon for the infantry, and was intended to bridge the gap between the maximum throwing distance of a hand grenade, and the lowest range of supporting mortar fire. An area of between 50 and 300 meters.

The M79 was a single shot, shoulder fired, break-barrel loading weapon which fired a spherical 40mm diameter grenade. 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 15 meters(?) of flight armed the grenade. 

Pages Taken From the Field Manual of the 40 MM Grenade Launchers, FM 23-31

For close range fighting the Army came up with two types of M79 rounds. The first was a flechette 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 the pellets could travel in their forward direction un-aided. The M79 could also fire smoke grenades, 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 in 25 meter 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 Later in the war the M79 was superseded by the M203 40mm launcher which was fixed beneath the fore grip of the M16 rifle.