Basically, the grenade launcher is a weapon which fires a grenade – a small shell, filled with high explosive or other agent, such as tear gas for less lethal application, bright burning compound for illumination purposes, incendiary filling etc. Of course, in most cases the grenade also must be fitted with a fuse, and with a safety, to avoid damage to the grenadier or handler. The simplest way to use the grenade is to throw it by hand; but the effective range and maximum weight of hand grenades is severely limited; so, at the earliest stages of the development of firearms, many armies used so called “hand mortars” – basically, the smoothbore muskets with short barrel of very large caliber, which was used to fire standard grenades at ranges beyond the limits of human throwing ability. During the First World War most nations started to use so called “rifle grenade launchers”. These launchers in fact were add-ons to standard issue military rifles, usually in the shape of a cup, attached to the muzzle of the rifle. A grenade was placed into this cup, primed, the rifle aimed toward the enemy, and then the grenade was launched using a special blank cartridge. This system, while enhancing the combat capabilities of infantry soldiers, has several drawbacks – for example in many cases the attached launching cup blocked the line of sight for the rifle.
There was another type of rifle grenade, which did not require any attachment to the rifle – instead, this system relied on a special thin rod, attached to a grenade as a tail. This rod was inserted into the bore of the rifle, and the grenade was launched using a blank cartridge. In either case, an attempt to fire the grenade with a standard round of riffle ammunition was disastrous to both weapon and the shooter. Most modern rifle grenade launchers got rid of both the cup launchers and rods attached to the grenade. Instead, these are just specially shaped muzzle devices, often also combined with flash hiders; the tail (rear) part of the grenade is shaped as a tube, which is slipped over the muzzle of the rifle. Also, most modern types of rifle grenade launchers use standard ammunition, and either trap the bullet and use its energy to project the grenade (helpfully known as the `bullet trap` type) or have a hole down the center through which the bullet escapes (the `bullet through` type), and use the gun gas expanding from the muzzle as a propellant. The latter loses something in energy, but gains through not having to switch the gas operation valve to `close` first.
The key problem with a rifle grenade is that when ready to fire, it effectively blocks the standard operation of the rifle. That means that if the shooter with a grenade in place has to fire his rifle in an emergency (e.g., if an enemy pops out in front of him), he should first either remove or launch the grenade, which will take time and may cost him his life.
To solve this problem, many countries developed and adopted so called “underbarrel grenade launchers”. Unlike the rifle grenade launchers, which are just attachments to the standard rifle, an underbarrel launcher is a complete weapon, with its own barrel, trigger / firing unit, safety, and often its own sights. The infantry [assault] rifle is used only as a host firearm, providing the stock for the grenade launcher. First developed between the wars in Italy and Japan, the underbarrel launchers appeared in their modern shape in the late 1960s, both in the USA and in the USSR. The underbarrel launchers do not block the rifle, but add a significant penalty in the bulk and weight of the combined weapon. Also, typical grenades for underbarrel launchers have warheads much smaller in size and weight, limiting their effectiveness against the targets (but increasing the number of grenades a soldier can carry with him).
The actual choice of the type of grenade launcher varies – some countries, most notably the USA and the former USSR/Russia, stuck completely with underbarrel grenade launchers, some others, like Belgium or France, seemed to prefer rifle launcher type, while many other countries, such as Germany, produced both types of weapon,.
The post-war period saw a short period of renaissance of the stand-alone grenade launchers, similar in basic idea to the “hand mortars” mentioned above. First these were re-introduced in service by the Germans during WW2, as the “kampfpistole” – a modified flare launcher, fitted with a rifled barrel and a detachable shoulder stock, and firing various types of grenades. In the postwar period, several countries developed single-shot, shoulder-fired grenade launchers, usually of 40mm caliber, which actually preceded the modern underbarrel grenade launchers and used the same types of ammunition. The most famous of these is probably the US M79 “thumper”, widely used during the Vietnam War. The key problem with these weapons was that they required the grenadier to carry some sort of personal defense firearm in addition to the grenade launcher, such as a pistol, submachine gun or rifle. Latter on, several countries produced multi-shot versions of stand-alone shoulder fired grenade launchers, usually in the form of a large revolver, or a pump-operated rifle with a tubular magazine. Military users mostly replaced these weapons with underbarrel grenade launchers, and stand-alone launchers are mostly used either by special operations forces or by police forces, which employ the launchers for less-lethal anti-riot applications, firing tear gas canisters and baton rounds (rubber projectiles or buckshot).
The most recent trend in this field is the development of time-fuzed grenades in conjunction with a fire control computer, mounted on the rifle and coupled with the sights. This unit incorporates a laser rangefinder, a ballistic computer and a means for programming the warhead before the shot. Before firing, the shooter determines the range to the target using the laser rangefinder, and the computer automatically corrects the sights to achieve the appropriate trajectory and presets the time fuze, so the warhead will explode when it reaches the target. This allows the engagement of targets `in defilade` (i.e. when they are hiding behind cover) by using air-burst fragmentation warheads. At the present time there are several projects that attempt to achieve such an effect, including the American XM-29 OICW system and French PAPOP. The Belgian F2000GL system offers a less costly alternative, with non-programmable grenades but with an electronic sighting unit which allows much more accurate long-range fire.
The key targets for rifle and underbarrel grenade launchers are enemy targets of the “soft” type – infantry, light entrenchments, unarmored or lightly armored vehicles etc. Most tanks developed during the Second World War and since are usually far too strong to be disabled with the relatively small amount of explosive carried in a typical grenade.