The recoilless rifle functions somewhat like a rocket launcher in that its projectile is propelled with a rocket charge that vents at the rear of the weapon. The RCL launcher pressurizes the charge only slightly, allowing the recoil of the exhausted propellant gases to be redirected forward to balance the recoil of the projectile. Hence the launcher is "recoilless." The backblast to the rear is very expensive in terms of propellant, reducing the range.
Unlike a rocket launcher, however, which fires fin-stabilized rockets from a smooth bore, recoilless rifle rounds resemble conventional artillery shells. They generally have a rifling band preformed to engage the rifled launch tube, and the "case" area of the shell is perforated to vent the propellant gases to the rear. The rocket stops burning when it leaves the tube, preventing injury to the operator. This system spin-stabilizes the projectile, hence the term "rifle."
The first recoilless rifle to enter service was the German Panzerabwehrwerfer 7,5 (or PAW), a 75mm gun developed to give airborne troops some useful anti-tank support before the Panzerfaust became widespread. The 75 was found to be so useful during the invasion of Crete that a larger 105mm version was developed. Interestingly both of these weapons were then copied almost verbatim by the US Army, reversing the flow of technology that had occurred when the Germans copied the Bazooka. PAWs remained fairly rare during the war, but the US versions of the 75 started becoming increasingly common starting in 1945. By the time of the Korean War they were found throughout the US forces.
As the wire guided missile became more and more popular in the late 1960s and early 1970s, the recoilless rifle started to disappear from the miliary. The last major use was the Ontos tank, which mounted six of the US 106mm on a light (9-ton) tracked chassis first developed for use by the US Army airborne troops in 1950. However the Army considered them useless, and the Marines picked them up instead, albiet only 176 of them. They used them to great effect as a fire support vehicle during the Vietnam War. The crews continued to report the Ontos was a very effective fighting vehicle in this role, but the military brass continued to argue for heavier designs, and in 1970 the Ontos was removed from service and most were broken up.
Today the recoilless rifle is mostly used in shoulder launched anti-armour systems, where they offer individual soldiers at least a fighting chance against even modern armoured vehicles. The most recent anti-armour shells feature modern tandem shaped charge warheads.
Old discarded weapons are still used by the US National Park Service as a system for avalanche control, an interesting case of swords to ploughshares.