The HK51 is not a gun that was ever actually produced by Heckler & Koch. It is instead a variation on the G3/HK91 originally developed by American H&K specialist Bill Fleming. He was contracted by a UK-based company called FR Ordnance to produce a submachine gun sized version of the G3 for British SAS and SBS use. Both agencies did actually buy a few examples, and used them a few times operationally. Some testing was done on specialty ammunition for them, but they were ultimately replaced by the rather more practical HK53 (essentially the same gun but in 5.56mm).

Where the HK51 saw most of its sales was on the US recreational market, with both Fleming and others making the guns, often in combination with registered H&K auto sears. With full-power 7.62mm NATO ammunition and all of an 8.3 inch barrel, the HK51 serves best as an attention-getter on a range full of other machine guns...