M79 40mm Grenade Launcher

Gary's U.S. Infantry Weapons Reference Guide

The M79 grenade launcher is a single-shot, break-open, breech-loading, shoulder-fired weapon. It consists of a receiver group, fore-end assembly, barrel group, sight assembly, stock assembly, and sling. The fore end assembly locks the barrel to the receiver. The stock is made out of fiberglass or wood and is attached to the receiver by a screw. A rubber recoil pad is attached to the butt of the stock to absorb some of the recoil.

Replaced by the M203 grenade launcher in U.S. service.

Operations include loading, unloading, and firing the weapon, which uses a high-low propulsion system to fire a round.

The firing pin strikes the primer, whose flash ignites the propellant in the brass powder-charge cup inside the high-pressure chamber. The burning propellant produces 35,000 psi (2,461 kg/cm²) 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 (211 kg/cm²) and propels the grenade from the muzzle at a velocity of 250 fps (76 mps).

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 grenadier loads and unloads the weapon with the barrel open and fires it from a closed bolt. The launcher must be cocked before it can be placed on SAFE.

Cycle of functioning:

1. Unlocking. Before you can unlock the barrel from the receiver and move the safety to the SAFE position, you must press the barrel locking latch lever all the way to the right. The spring-loaded latch lock holds the barrel locking latch open.
   
   
2. Cocking. Opening the barrel cocks the weapon by causing the cocking arm to lift the cocking lever. The cocking lever rotates around the hammer pin until it contacts a stud on the hammer. Then the lever rotates upward with the hammer until the sear engages the sear notch, cocking the weapon.
   
   
3. Extracting. Extraction occurs while you are cocking the weapon. As you open the barrel, the spring-loaded extractor withdraws the spent cartridge case about 1/2 inch from the breech end of the barrel.
   
   
4. Ejecting. The M79 grenade launcher does not eject rounds automatically; you must remove the expended cartridge case or live round from the barrel.
   
   
5. Loading. With the barrel in the open position, insert a cartridge into the breech end of the barrel.
   
   
6. Chambering. Closing the barrel forces the extractor into the extractor housing, which causes the cartridge to seat in the chamber.
   
   
7. Locking. Closing the barrel also depresses the latch lock, which rotates until it locks the barrel to the receiver (when it engages the barrel locking lug). Before firing the weapon, you must push the safety forward to expose the letter "F."
   
   
8. Firing. As you pull the trigger rearward, it rotates on the trigger pin. The rear of the trigger lifts the rear of the sear, causing the nose of the sear to disengage from the sear notch in the hammer. This releases the spring-driven hammer, which strikes the firing pin and drives it forward to strike the primer of the cartridge. When you release the trigger, the hammer settles back slightly, allowing the firing pin spring to withdraw the pin from the face of the retainer.

Ammunition for the M79 is issued in the form of a complete, fixed round. Click here for more information.

The launcher may be fired from standing, kneeling, or prone position. When firing long range from the prone position, place stock of weapon on the ground. For all other direct-fire positions, hold the stock firmly against your shoulder.


At ranges less than 164 yards (150 m), you can fire normally from your shoulder in any position. However, to maintain sight alignment at greater ranges, lower the position of the stock on your shoulder or drop the butt from your shoulder. At near-maximum ranges, you must position the stock between your waist and your armpit and hold the stock firmly against your body with your upper arm. In the prone position, once the stock is no longer against your shoulder, rest the butt of the launcher on the ground. Be careful to keep your head level when your cheek breaks contact with the stock.

Hold your right thumb against the right side of the grenade launcher's stock. If you place this thumb over the small of the stock, the safety can injure your thumb, and you will not achieve a spot weld with the grenade launcher.