A rifle grenade is a projectile designed to be fired from the muzzle of a weapon or a special adaptor, using the firing of the weapon to either propel the projectile directly or ignite a propelling charge. They differ from modern unitary grenade rounds used by grenade launchers in that they are not self-contained rounds and require some operation from the parent weapon in order to fire.
The term is not a reference to the parent weapon: it is not uncommon for rifle grenades to be launched from shotguns.
Experiments in firing grenado fuse bombs from specially made wide-bore guns called "hand-mortars" date back as early as 1472. Due to the requirement to light the fuse before loading and their tendency to force the fuse into the bomb and detonate it instantly on firing, these were never particularly practical weapons.
The first experiments with modern rifle grenades are generally dated back to work in 1904 by a Japanese Colonel named Amazawa, who experimented with rifle-launched grenades during the siege of Port Arthur. Previously, the throwing of hand grenades was limited to the largest and strongest men in a unit, and it was reasoned that a launching from a firearm would permit weaker soldiers to employ grenades as well. The general idea behind the rifle grenade was to allow soldiers to turn their rifle into an ersatz mortar at short notice.
As the idea evolved, the rifle grenade quickly exceeded the effective range of a thrown grenade, but never directly displaced the hand grenade. Instead, it became an intermediate measure between the hand grenade and the light mortar.
The "rod grenade" was developed in Britain in 1907 by Martin Hale. This was a very simple system where a long caliber-sized metal rod was attached to the base of a standard hand grenade, launched using a high-powered "grenade blank" cartridge after pulling the pin: a metal band muzzle device was used to retain the grenade's safety lever prior to launching. While this system was functional, it was rather dangerous, since if the metal rod became lodged in the barrel the gun would explode. Damage to the rifle from firing grenades would quickly render it useless as a regular firearm.
The French V-B, first adopted in 1916, was the first cup-type rifle grenade launcher. In this type, an adaptor shaped like an open food can is placed on the muzzle of the parent firearm, or in some cases mounted underneath, above or alongside it using a gas trap system. This serves as a muzzle-loaded barrel, allowing the grenade to be seated inside it. The V-B was also the first rifle grenade designed to be fired using standard ammunition: it was a "shoot though" type, with a tubular path through the grenade body to allow the bullet to pass through it, arming it as it did so. Cup-type launchers could fire either specially designed projectiles, or regular grenades if the cup was large enough and the grenade fitted with a gas check disc on the base.
During WW2 another role was added: as well as being used to launch fragmentation grenades, rifle grenade launchers were used to launch hollow charge warheads for use against fortifications and armored vehicles. The British No. 68 AT Grenade, adopted in 1940, was the first hollow-charge weapon used in combat. Rifle grenades were also produced for other functions, such as deploying parachute flares and WP smoke/incendiary warheads.
The US adopted the first muzzle-mounted launchers prior to WW2: these were based on spigot mortar mechanisms, using a 22mm adaptor on the rifle's muzzle as the spigot. A socket in the base of the grenade's tailfin assembly mated with the adaptor. On bolt-action rifles this allowed the adaptor to be fitted at all times without interfering with the weapon's functionality, though the version for the M1 Garand had to disable the rifle's gas system to prevent explosions. The muzzle-mounted grenade had an advantage over the cup-type in that the warhead could be made as large as was desired for the intended use, much like the rod grenade, without the mechanical damage associated with the latter type.
Clip-on tailfin assemblies called "grenade projection adaptors" were produced for firing standard hand grenades from the American launchers. These adaptors, the M1 for fragmentation grenades and the M2 for smoke grenades, featured a clip that retained the safety lever of the grenade after the pin was pulled: during launch, inertial forces would release the clip and allow the grenade to arm. The postwar M34 WP grenade was specifically designed to fit into the conical cup of the M1A2 rifle grenade adaptor.
During WW2 it became clear that the rocket launcher and recoilless rifle were far superior methods of delivering hollow charges. However, a final generation of heavy AT rifle grenades were produced while the technology matured enough to equal rifle grenades in terms of portability, many using a new propulsion technique called "bullet trap." Like shoot-through, this method did not require the use of grenade blanks, but rather than compromise the warhead with a hole down the middle, it is designed in such a way as to stop the bullet within the body of the grenade. NATO standardized the US-style spigot muzzle adaptor for combat rifles, simply incorporating the ability to launch 22mm rifle grenades into the weapon's normal flash hider. Gas-operated rifles of the period often have a specific regulator setting for launching rifle grenades.
With the advent of disposable anti-tank launchers such as the M72 LAW the anti-tank rifle grenade was consigned to history. Similarly, the development of grenade launchers using unitary cartridges led to the obsolescence of the HE rifle grenade. Underbarrel launchers like the M203 grenade launcher and GP-25 were far preferred in military use, in particular because they proved more accurate and their HE ammunition was just as effective while being much lighter.
Since they allow an over-caliber projectile, rifle grenades continued to be employed in riot control operations for smoke and gas rounds. A recent generation of modern rifle grenades has been produced as well, mostly as breaching weapons or for other special purposes. Most modern Western military rifles retain the ability to fire rifle grenades, as there is no real tradeoff to having a 22mm flash hider with the required features to attach a grenade to it.
Fundamentally, any rifle grenade involves using propellant gas from the parent weapon to launch a large projectile from the weapon's front end, either though gas pressure alone or through the ignition of a booster charge in the grenade body.
Most early rifle grenade systems were aimed intuitively based on extensive training, but sighting systems were developed over time. Often these proved overly complicated and were seldom issued or used, as with the rare M15 grenade sight for the M1 Garand. The most practical sights developed for rifle grenades were flip-up ladder types, which were incorporated into some later weapons such as Yugoslavian SKS rifles.
The heavy recoil from rifle grenades means they are generally fired with the stock of the parent weapon rested on the ground. While the kick is fierce, most launchers can be used with the stock under the user's arm or even fired from the shoulder, though launchers using heavy grenades or range-boosting charges can be dangerous to fire in this way. The Soviet Dyakanov launcher was an unusual low-angle example that was instead fired using a heavy bipod.
Another unusual example which borders on being in a class of weapons all of its own is the Brunswick RAW (rifleman's assault weapon). This was a gas-trap type underbarrel adaptor with a spherical HESH warhead the size of a cantaloupe melon, with its strangest feature being that it was rocket-powered rather than launched with a conventional propelling charge. It was evaluated in the 1970s, but only saw limited service with the US Marines before being discarded in favor of the Mk 153 SMAW: the Army rejected it outright, focusing on the doomed FGR-17 Viper program.