The Favor submachine gun was originally designed in France during World War II by Colonel Paul Favier, from which it got its name, and was apparently due to be developed at MAT before the plans were stolen. After the war, Col. Favier took the concept to Switzerland and sold it to the Swiss firm of Rexim, Geneva. A batch of 5,000 guns was produced in 1954, but Rexim encountered difficulties in marketing the weapon and consequently it did not do well financially, except for a small sale to Turkey. In 1957, Rexim went bankrupt and ceased to exist, and the production rights were then transferred to Coruña Arsenal in Spain, where it was produced as "La Coruña". It was tested by the Spanish Army but rejected, and by the end of the 1950s, the Favor had been abandoned.
Some Favor submachine guns were trafficked to the Congo during the civil war there. The gun was also produced under license in Turkey for a short time.
The Favor was a blowback-operated submachine gun of a unique design. It was fired by a hammer which did not resemble a hammer in the traditional sense, but was in fact a hollowed-out cylinder that wrapped around the bolt. The hammer was held back by a small sear. The return spring was in two parts: an inner spring that operated the bolt, and an outer spring that operated the hammer. The firing pin was retracted by a spring connected to a yoke that protruded from either side of the bolt. When the weapon was cocked, the bolt would travel backward to allow a new round to be fed from the magazine and then the bolt would come forward again to push the bullet into the chamber. When the trigger was pulled, it would release the sear holding back the hammer and the hammer would fly forward until it struck the yokes protruding from the bolt, which would force the firing pin forward to strike the primer of the chambered bullet. The blowback would then send both the bolt and the hammer flying backward and the process would repeat. The firing pin retracted back into the bolt when not in contact with the hammer, preventing accidental discharge.
The Favor was designed with a fire selector and if set to semi-automatic fire, the bolt would return forward after firing but the hammer would be caught by the sear after every shot, so it would not strike the firing pin again. The system worked well and was mechanically impressive but was not seen to offer any advantage over conventional blowback designs.
The build quality of the Favor was good and it was constructed primarily from high-quality steel stampings. It was fitted with a folding skeleton stock and a perforated barrel jacket, with a four-slot compensator at the muzzle. A folding spike bayonet came standard. The Favor also had the unusual ability to fire rifle grenades at a distance of 110m. The rear sight was an adjustable tangent to assist with aiming the grenades.