The MAS 1951 (French: Fusil Automatique Manufacture d'Armes de Saint-Étienne 1951), commonly known as the MAS 51, was a prototype French battle rifle designed in 1951 and produced in very small numbers in prototype form only by the Manufacture d'armes de Saint-Étienne. One of a number of automatic weapon prototypes designed by MAS during the 1950s and 1960s, the MAS 51 was an unsuccessful attempt at replacing the MAS 49 in service.
The MAS 51 was one of a number of automatic weapon prototypes designed by engineers at MAS to replace the MAS 49 rifle in service; it was felt that there was a need for the French to standardize their weapons with the then-newly introduced 7.62×51mm NATO cartridge, and to reduce or avoid reliance on other countries for equipment the weapon had to be of French manufacture.
The MAS 51 was made in three variants; a carbine, bullpup layout and full-length rifle. The weapon was never actually submitted to military trials. While the rifle's attempt at replacing the MAS 49 was without success, the work done on the MAS 51 eventually culminated in the design of the FA-MAS Type 62.
The MAS 51's design was heavily based on the MAS 49, being gas-operated with a tilting bolt. The MAS 51's charging handle was on the left of the receiver.
Conventional and bullpup versions were made.