FR Ordnance MC51
The picture posted features a FR ORDNANCE MC51.
Please note that MC51 is NOT, was NEVER, and will NEVER BE, manufactured by HECKLER & KOCH. The MC51 was an early-mid '80s product of FR ORDNANCE, a BRITISH company, which contracted the famous gunsmith BILL FLEMING to develop a shortened version of the HECKLER&KOCH G3 in order to equip the British special forces (SAS, their equivalent of the USA "Delta Force", and SBS, their equivalent of the USA "Navy SEALS") with an handy but POWERFUL weapon. Both SAS and SBS, although they were phasing out their FAL rifles in order to get 9mm MP5 SMGs and lighter 5'56-NATO rifles and carbines, still needed (and they STILL USE) some 7'62-NATO weapons for covering actions and in order to have more stopping power.
BILL FLEMING developed something very similar to the MP-5 SMG design, a few inches longer only. This was done A FEW YEARS BEFORE the rollout of the HECKLER&KOCH HK53. It uses a 20-round 7.62-NATO magazine. It can also use the 30-round magazines used in the old L1A1-FAL rifle), a collapsing stock and a S-E-F trigger.
Three versions of the MC51 were developed: the standard one (you see it in the picture), an SBS version (either with fixed or collapsing butt stock, and an integrally suppressed barrel, as the MP5-SD's one), and a tactical version with a fixed stock and a 3-rounds burst limiter. Although the MC51 went out of FR ORDNANCE facility in very small numbers, all of them shipped to the UK special forces, and most of them phased out in a few years and replaced by the HK53.
I wish to repeat that, despite of the common error mainly found on the web in airsoft weapons sites, the MC51 was NOT a German or a Heckler & Koch product, but came from the FR ORDNANCE facilities in the UNITED KINGDOM.
Additional information from our U.K. Arms Expert.
The MC-51 did exist, although I have never handled one, preferring to keep what is left of my hearing. SAS had them in service for a very short time indeed, but problems with keeping them functioning with the reduced gas volume due to the shortening of the cylinder and the appalling recoil, bad enough on the original G3, but truly vicious on this much shorter weapon, meant that very few were ever used in any kind of action. They are not now in use of any kind, and are certainly not the preferred weapon of support. For that purpose the SAS uses either their own version of the Spanish Ameli or the FN Minimag. (Also the MC51 was made in the USA.)
FR Ordnance MC51
Picture and information courtesy of: Pierangelo Tendas