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Airsoft-version of MC51 by Tokyo Marui

The FR Ordnance MC51 was a compromise between an assault rifle and a submachine gun. It was developed in the mid-80s by famous gunsmith Bill Fleming for the British firearms manufacturer FR Ordnance. The weapon was based on Heckler & Koch's G3 battle rifle. Contrary to the popular belief, the MC51 was not manufactured nor developed by Heckler & Koch, although the 51 in its name comes from H&K old numbering system (the first 5 indicated a select-fire SMG, while the 1 indicates its chambering for the 7.62mm NATO round).

MC51 was manufactured for the British SAS and SBS, who required a compact but powerful weapon, for situations in which the stopping power and armor piercing capabilities of 9 mm SMG round were inadequate. All weapons produced were shipped to the UK special forces. Most of them were replaced in a few years by HK53.

Basically MC51 was a shortened version of the G3, using the same 7.62x51 mm round. It was very similar to H&K MP5 in design, but had a longer G3-type receiver. MC51 used a 20-round 7.62 NATO-magazine and was also able to use an old 30-round FN FAL-magazine. It had a collapsing stock similar to H&K G3A4 and an S-E-F triggergroup.

MC51 was produced in three variants:

• Standard variant, described above.
• SBS variant, which had either a fixed or collapsing stock and an integral suppressor, similar to the one in H&K MP5 SD.
• Tactical variant, with fixed stock and a 3-round burst capability.

MC51 was not a successful weapon and was used by SAS for a very short time. The reduced gas volume due to the shortening of the cylinder caused problems in keeping the gun functioning, and called for punctual and extensive checks and maintenance almost every time after the gun was fired. The heavy recoil of the 7.62x51 mm round was also vicious in a weapon this short, making the gun very hard to control in full automatic fire.

The MC51 has been discontinued from production and there are no official records of it ever being used by any other organisation than British SAS and SBS.

Allegedly, the MC51 was also being used by the Joint Anti-Smuggling Task Force (Joint British Army, RN, RHKP, RHKAAF/GFS and HK Custom unit) and Special Duties Unit (RHKP) in Hong Kong during the early 90's, to combat extra large size armoured smuggling speedboats.^{[citation needed]}

Lately, a UK-based company called Imperial Defence Services Ltd. claims to be producing the MC-51 (standard variant), and lists it on its website. In addition, a weapon similar to the MC51 called the M41 Offizier is produced by Schwaben Arms GmbH of Germany, which used to manufacture the G3 rifle under license from Heckler & Koch.

Tokyo Marui produces an airsoft version of the standard model of MC51 and a version called the G3 SAS, which is similar to the MC51, but without a stock and with a larger foregrip and an RIS rail on the bottom.

• MC51 on Securityarms.com
• MC51 on the website of Imperial Defence Services Ltd., possibly the current manufacturer