Staff Writer (Updated: 11/11/2016):
The XM79 was a break-action single-shot weapon featuring a wooden stock, integrated wooden grip and wooden forend. To this was inlaid the metalwork required of the action and the barrel which was of a simple, smooth-sided finish. The trigger unit was underslung at the action and ahead of the grip in the usual way. A forward iron sight was added at the muzzle while a more useful flip-up leaf assembly was added ahead of center for ranging. The operator need only to break the action open, insert a 40mm projectile into the breech, close the action, aim and fire. Overall weight of the loaded weapon was a handy 6.45lbs with a manageable unloaded weight of 6lbs. Overall length was 29 inches which made for a relatively compact weapon system. The barrel measured 14 inches long.
The XM79 passed the requisite trials to become the "M79" in the US military inventory. The series was adopted in late 1960 and serial production spanned from 1961 into 1971 to the tune of 350,000 examples.
The ability to evolve the infantryman into a grenadier held its advantages in that it provided ranged artillery-type fire at the squad level. However, its dedicated nature meant that the operator was issued the weapon as his primary with a sidearm pistol or bayonet as his only means of close-quarters protection, a limitation which soon became wholly apparent to observers. This led to the experimental Colt XM148 underbarrel system that was eventually abandoned. The XM148 and M79 were eventually superseded by the underbarrel M203 series in modern times.
The M79 relied on the proven 40x46mm grenade which was developed into a variety of useful warhead forms to suit specific battlefield situations. There was the standard HE (High-Explosive) type followed by a general smoke and illumination round. To this was added anti-personnel types including flechette-based rounds, buckshot projectiles and non-lethal rounds. A trained M79 grenadier could lay down an optimal rate-of-fire of approximately six rounds per minute. Each grenade exited the muzzle at 247 feet per second and was based on the US Army's called-for "High-Low Propulsion System" to ensure the recoil forces could be controlled from a weapon this small. Effective useful range was within 380 yards while a maximum range of 440 yards could be reached in ideal conditions. However, each grenade required a minimum 30 meters for the activation system in each projectile to achieve arming.