NATO Standardisation
With the creation of NATO in 1949, there was a clear preference that NATO forces would have commonality of weapons and ammunition, so weapons designs had to meet with the approval of more than one government, and it was hoped, would be adopted by the organisation as a whole. It was at this point that the US put forth its own designs for NATO standardisation, using the 7.62x51mm round in their prototype T25 and T44 rifles. Matters came to a head in 1951 in a shoot-off conducted at the Aberdeen Proving Grounds, with the US claiming the British round was underpowered, and the British claiming the US round was too powerful to be used in a rifle in full-automatic mode. At the same trials the Belgian .280 FN FAL was also tested. A series of lengthy debates followed, which were finally settled in an unlikely fashion when Canada stated they would use the British .280 round, but only if the US did as well. It was clear this would never happen. Winston Churchill felt a NATO standard was more important than any qualities of the weapon itself and overturned the decision by the previous Labour Minister of Defence, Manny Shinwell, who had already announced an intention to move to the .280 and the EM-2. During this time, prototype EM-2s were built in several different calibres. Chambons built two for the 7x49mm "Second Optimum" cartridge and another two for the 7.62x51mm. One of the Chambon prototypes was even rebuilt for the US .30-06 cartridge. RSAF-Enfield and BSA built 15 and 10 prototypes for the 7.62x51mm, respectively. Canadian Arsenals Limited also built 10 rifles for the 7x51mm "Compromise" cartridge.
It was clear the EM-2 could not be easily adapted to the powerful 7.62x51mm round, so the only realistic alternative was to adopt a licensed version of the FN FAL from Fabrique Nationale. The FAL was itself a re-designed version of FN's own design for an intermediate cartridge rifle using the .280 round (first protoypes had used the 7.92x33mm Kurz German round from the MP44). However, the FAL was more easily adaptable to handle the more powerful, longer round. It grew in weight and length as a consequence. Churchill had hoped that with the British Commonwealth and other NATO countries adopting the FN FAL, the US Army would do so as well. In the end, the US adopted the T44 as the M14, which had won US trials against the more radical T25 experimental weapon.
In time, the British position on intermediate cartridges was vindicated. The 7.62x51mm was too powerful to be controllable in automatic fire and smaller cartridges were proven to be necessary for that use. Due to its experience in Vietnam in the mid-1960s, the US adopted the AR-15 as the M16. Chambered for the 5.56x45mm intermediate cartridge, the M16 displaced the M14 which had been adopted little more than a decade earlier. Some years afterwards, NATO also agreed to move to a smaller round more suitable for full-automatic fire and lighter weapons, ultimately adopting the 5.56x45mm. Prior to committing to the 5.56mm, the British Army conducted additional intermediate cartridge research. Around 1970, a pair of the original .280 EM-2 were re-barrelled for an experimental 6.25x43mm cartridge. However, the revival of the EM-2 was short-lived as the 6.25mm was dropped for an even smaller 4.85mm cartridge in a completely new rifle, the L64/65 which evolved into the 5.56mm SA80 currently used by the British Army. In spite of having a similar appearance due to its bullpup layout, the SA80 is not mechanically related to the EM-2; rather it is essentially a bullpup adaptation of the AR-18/SAR-87. However, the idea of an Infantry Personal Weapon which lead to the design of the EM-2, was reflected in the SA80 system.
Read more about this topic: EM-2 Rifle