The 5.56mm NATO round, pronounced "five-five-six", is an intermediate cartridge and the main bullet type used by guns made for NATO militaries. It was first used in the Vietnam War as the main bullet used by the US military.
Early in the Vietnam War, the main gun used by American soldiers was the M14. Many soldiers complained that the 7.62mm NATO round used by the M14 was too heavy and that the gun kicked back too hard to shoot accurately when shooting in full-automatic fire. To solve this problem, the US military decided replace the M14 and the 7.62mm NATO by creating a new assault rifle that would shoot lighter bullets with less kickback. That gun would be the M16A1, the first standard issue gun to fire the 5.56mm NATO bullet.
Even though the M16A1 had many design flaws that would cause the gun to jam, the 5.56 bullet performed well in battle. The 5.56 did so well that the Soviet Union made its own intermediate cartridge, the 5.45 x 39mm, to compete with NATO.