Not something you want to ND.
Grenade launchers break down into two categories, individual and crew served.
Prior to the adoption of a true grenade launcher, a 'rod' type grenade had been used that had a steel shaft that was inserted down the barrel of a rifle - usually a worn out example with its fore-end bound with reinforcing wire. This was replaced by a cup type discarger used in WW-1 and early WW-2 on Lee Enfield No1 rifles ("SMLEs"). Ammunition originally an adapted Mills Bomb, but later one of the first HEAT grenades was issued.
A detachable spigot type launcher was subsequently adopted late in WW-2 for the No4 rifle to fire US M9 HEAT rifle grenades. The war ended too soon and this launcher came into service in the early 1950s with the No94 (Belgian MECAR ENERGA) HEAT AT grenade. A rifle grenade adapter consisting of a tail boom with fins and a ballistite cartridge was designed for the L1 series hand grenade, but appears not to have entered service. A similar detachable launcher served on the L1A1 rifle until the ENERGA was replaced by the L1A1 66mm LAW circa 1970 (launchers were retained by the navy for line throwing).
In the meantime the US had adapted the 'high-low' pressure principle used in a German WW-2 AT gun to create a grenade cartridge that could project a 40mm grenade to circa 400 metres with great accuracy from a relatively lightly constructed weapon. The original weapon, the M79, was designed to resemble a break action shotgun in operation. Small numbers of this weapon served with UK forces - allegedly they were used to protect permanent VCPs as late as the 1980s. It was found that dedicating one soldier to grenade launcher use was tactically restrictive, so a call went out for a launcher that would fit on an M16 rifle. The resulting Colt CGL, adopted as the XM-148, and used in small numbers by the SAS, proved fragile and unreliable. A better design, the M203, was subsequently adopted and produced in great numbers (ironically by Colt). The M203 served in the UK Armed forces in greater numbers, but was never general issue. It continues in use with those units with M16 or Canadian C7 rifles.
Rifle grenades made a return for use with the L85A1 rifle - itself permanently fitted with the standard NATO 22mm grenade spigot. Initially a rocket-assisted HEAT AT grenade was procured for the point defence of permanent VCPs. However, French made rifle grenades of more conventional appearance were issued on a wider scale for Operation Granby. These were subsequently replaced by an Israeli version and subsequently by a 'shoot through' telescopic rifle grenade. However, an underslung grenade launcher designed by Heckler and Koch has now been issued for the L85A2, chambered for the same rounds as the US M203. Issued on a scale of two per section, this will presumably replace all existing rifle grenades.
In Op Granby US Mk19 40mm grenade launchers were used from vehicles by the SAS. The Mk19 fires similar projectiles to the M203 but from a longer (53 vs 46mm) and higher pressure cartridge, giving a vastly greater range of circa 2200 metres. The Mod 3 version of the Mk19 fires at 325-375 RPM. As of 2005, crew served grenade launchers were meant to be becoming a standard item in UK infantry battalions, though I do not yet know which one or at what scale of issue. Anyone?
Never ones to miss out on a good idea the Soviets also developed a belt fed crew served fully automatic grenade launcher although in the more reserved 30mm calibre. Credited with turning around the casualty figures for ambushes in Afghanistan (3 dead muj for every dead Red Army squaddie) the AGS-17 with it's maximum range of 1750 metres and ability to provide fully automatic indirect fire became something of a favourite for whoever was writing training scenarios at Sandhurst in the late 90's. For the less weapon mad Ocdts the prospect of 'something like a machine gun, but with grenades' was concerning to say the least. The 19 round magazine does translate into quite a lot of potential for hurt but, thankfully for those who might find themselves on the wrong end of one, not the same unrelenting torrent of pointy metal and HE hurt that the immense box on the side of a Mk19 allows.