The remainder of the SA80 family comprises the L86 Light Support Weapon, the short-barrelled L22 carbine and the L98 Cadet rifle. The SA80 was the last in a long line of British weapons (including the Lee–Enfield family) to come from the Royal Small Arms Factory, the national arms development and production facility at Enfield Lock.
The system's history dates back to the late 1940s, when an ambitious programme to develop a new cartridge and new class of rifle was launched in the United Kingdom based on combat experience drawn from World War II. Two 7mm prototypes were built in a bullpup configuration, designated the EM-1 and EM-2. When NATO adopted the 7.62×51mm NATO rifle cartridge as the standard calibre for its service rifles, further development of these rifles was discontinued (the British Army chose to adopt the 7.62 mm L1A1 SLR semi-automatic rifle, which is a licence-built version of the Belgian FN FAL).
In 1969, the Enfield factory began work on a brand new family of weapons, chambered in a newly designed British 4.85×49mm intermediate cartridge. While the experimental weapon family was very different from the EM-2 in internal design and construction methods, its bullpup configuration with an optical sight was a clear influence on the design of what was to become the SA80. The system was to be composed of two weapons: the XL64E5 rifle (also called the Enfield Individual Weapon) and a light support weapon known as the XL65E4 light machine gun.
The sheet metal construction, and the design of the bolt, bolt carrier, guide rods, gas system and the weapon's disassembly showed strong similarities to the Armalite AR-18 which was manufactured under licence from 1975 to 1983 by the Sterling Armaments Company of Dagenham, Essex, and which had been tested by the UK MoD in 1966 and 1969.
During the development of the SA80, a bullpup conversion was made of an AR-18 and a Stoner 63 at Enfield due to the fact they could be used with stocks folded/without stocks which allowed the bullpup conversion and were later chambered in the experimental 4.85x49mm round. A bullpup conversion of the AR-15 was previously considered but the buffer tube in stock prevented the idea.
Technically, in the mid-1970s, the 4.85×49mm round was seen as superior to the then existing version of 5.56mm M193 round in use by the US (for the M16/M16A1) and by other forces. (This was the expressed view of trials team members whilst demonstrating the XL64E5 prototype at the British Army School of Infantry at Warminster.) It should be noted that development of small-arms munitions have a long and continuous life and it was estimated by the trials specialists from Enfield that this weapon would ultimately be superior in the 4.85mm configuration. For the 4.85 mm round, both propellant and projectile were at the beginning of their respective development curves. Also, weight for weight, more rounds of ammunition could be carried by an individual soldier – a considerable advantage on the battlefield. It was regarded as probable at the time that the argument for the 5.56 mm standard within NATO had more to do with the economics involved. Over the lifetime of a small-arms weapon type, far more money is spent on the munitions than the weapons themselves. If the 5.56 mm supporters had lost the argument in favour of a British 4.85 mm round, the economic impact would have been very large and political pressure undoubtedly played a part in the final decision.
In 1976, the prototypes were ready to undergo trials. However, after NATO's decision to standardise ammunition among its members, Enfield engineers re-chambered the rifles to the American 5.56×45mm NATO M193 cartridge. The newly redesigned 5.56 mm version of the XL64E5 became known as the XL70E3. The left-handed XL68 was also re-chambered in 5.56×45mm as the XL78. The 5.56mm light support weapon variant, the XL73E3, developed from the XL65E4, was noted for the full-length receiver extension with the bipod under the muzzle now indicative of the type.