 The US was slow to follow suit, insisting on retaining a full-power, 7.62×51mm NATO rifle, the M14 (although this "was" selective fire), until too-hastily adopting the 5.56mm M16 rifle in the mid-1960s, with initially poor results due to the rapidity of its introduction (but later to become a highly successful line of rifles and carbines).In the 1950s, the British developed the .280 British, an intermediate cartridge, and a select-fire bullpup assault rifle to fire it, the EM-2.They pressed for the US to adopt it so it could become a NATO-standard round, but the US insisted on retaining a full-power, .30 caliber round.