The MC-51 was a battle rifle designed and manufactured by UK-based firearms company FR Ordnance.
During the mid-1980s, UK special forces explored possible alternatives for the Sterling L34 silenced submachine gun, which had poor range and stopping to replace the Sterling. The MC-51 was essentially a very short G3 rifle, cut down to the dimensions of a submachine gun. It was chambered for 7.62x51mm NATO ammunition, which not only offered greater power than the 9mm Sterling, but also enabled far superior range and accuracy. The MC-51 was not much larger than the Heckler & Koch MP5, a weapon that British special forces had already adopted. However, problems with the MC-51 soon emerged: the full-power ammunition meant that recoil was incredibly strong, and the MC-51 was overall not a very reliable weapon. It was adopted with British special forces such as the SAS for a very limited period, and other designs, such as the 5.56 HK53, proved more reliable.
In all, about 5 different variants were produced. The standard MC-51 was available with both a telescoping stock and a fixed stock, the latter designed for use with the Special Boat Service (SBS). There was also an MC-51SD, which had an integral suppressor, much like that of the MP5SD. The MC-51SD was also available with both a telescoping stock and a fixed stock. The last variant was the very compact MC-51K, which was not unlike the MP5K. Recoil issues were even more persistent for the MC-51K than the regular MC-51.