"HK51" is a blanket term given to a custom variant of the G3 or HK91 which has been shortened to be comparable in length to the MP5. The first factory produced variant of this type was the MC51 which was produced by FR Ordnance for British Special Forces use. It was produced in several different variants including the standard model (with fixed or retractable stock), the silenced MC51SD (with fixed or retractable stock), the shortened MC51K, and a "Tactical" model with the full auto setting replaced with a burst position and a fixed stock. Although it was adopted by the British SAS and SBS, it was quickly replaced in part due to reliability issues and the impracticality of firing a cartridge as powerful as the 7.62x51mm NATO from such a short barrel, but also due to the fact that Heckler &