Grenade launchers can either come in the form of standalone weapons (either single-shot or repeating) or attachments mounted under the barrel of a rifle. Some rifles have been designed to fire rifle grenades, either from their muzzle or from a detachable muzzle-mounted launcher. Larger grenade launchers may be mounted on vehicles.
Most grenade launchers as of 2014[update] are man-portable, shoulder-fired weapons issued on a squad level, though larger launchers are sometimes mounted on armored vehicles. The most common grenade-round in use by modern militaries is the 40 mm fragmentation grenade, which is effective against a wide range of targets, including infantry and lightly armored vehicles. The ability of the grenade launcher to loft payloads in a high arc has resulted in many "specialty" grenades such as less-lethal sponge grenades, flares, and even a video camera that surveys the battlefield from a bird's-eye view.
Heavier grenade launchers, such as automatic grenade launchers, are typically mounted on vehicles or in emplaced positions. These generally resemble a large machine-gun intended to rapidly launch grenades to suppress enemy activity in a target area.
Many grenades have been designed to launch from a rifle's muzzle, usually using either a special blank propellant cartridge, or more modern "bullet trap" and "shoot through" types which allow the grenade to be fired using live rounds. This system has two key advantages: the grenade can generally be made larger and more powerful as compared to underbarrel or standalone weapons, and the rifle's weight and handling characteristics are not affected as with underbarrel systems.
The disadvantage of this method is that when a soldier wants to launch a grenade, he must mount the grenade to the muzzle prior to each shot. If he is surprised by a close-range threat while preparing to fire the grenade, he has to reverse the procedure before he can respond with rifle fire. Rifle grenades also tend to be more difficult to fire accurately compared to under-barrel or standalone designs.
The man-portable grenade launcher can come in the form of either a single-shot weapon or a repeating weapon resembling a large revolver or pump-action shotgun. Examples include the M79 (single-shot) and the Milkor MGL (repeating). They fill the gap between the hand grenade and the mortar. Modern developments tend toward faster-firing grenades with a smaller blast radius to reduce collateral damage. The XM25 CDTE is a shoulder-fired, magazine-fed semi-automatic launcher firing 25 mm projectiles. It was originally a component of the XM29 Objective Individual Combat Weapon program, but modified to a larger caliber. A 12 Gauge grenade round called the Frag-12 has also been developed for the Atchisson Assault Shotgun.
Since grenade launchers require relatively low internal pressure and only a short barrel, a lightweight launcher can be mounted under the barrel of a traditional rifle. This reduces the weight the soldier must carry by eliminating the grenade launcher's buttstock and makes the grenade launcher available for use at a moment's notice. Underbarrel tubes generally have their own trigger group; to fire, one simply changes grips, disengages the safety, and pulls the trigger. In Western systems, the barrel slides forward or pivots to the side to allow reloading. Soviet/Russian launchers are instead loaded from the muzzle, with the cartridge casing affixed to the projectile in the style of a mortar shell. For aiming, attached grenade launchers typically use a separate sight attached to the rifle's frame alongside the iron sights, or attach a flip-up sight directly to one of the rifle's sights. Examples of modern attached grenade launchers are the M203, GP-30, AG36, and FN40GL which mount to service rifles.
A late development is the 3GL from Metal Storm. As with most Metal Storm products, this weapon contains three electrically ignited grenades stacked front-to-back in a single tube to eliminate reloading.
Automatic launchers include the Mk 19, AGS-17, and the HK GMG, which all fire at a higher velocity than related shoulder-fired grenades. They generally function as large-caliber machine guns with a relatively low rate of fire, used from an emplaced position in a similar way to a heavy machine gun. The heavy equivalent of the XM29 is the XM307 ACSW automatic grenade launcher that is easily convertible between the 25 mm grenade ammunition and standard .50 BMG cartridges. Both are intended to fire programmable "smart" grenades capable of being set to explode at a certain distance from launch or at a certain height above the ground. This gives the ability to hit targets inside rooms or behind hard cover that would normally not be reachable by small arms fire.
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