This article is about HK51. You may be looking for other similar named Heckler & Koch firearms.
"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 & Koch very soon afterwards produced the HK53 which was similar in dimensions (actually slightly smaller and lighter) but fired the much more practical 5.56x45mm NATO round. FR Ordnance contracted the gunsmith Bill Fleming to design this variant, who later marketed similarly modified versions of the G3 in the US as the "Fleming HK51", and it has since gone on to be produced by several different companies.