The US Army issued the M-16 automatic assault rifle during the Vietnam war. The troops appreciated the increased firepower, but units in action soon began to run into ammunition supply problems. A soldier could fire of a 20 round magazine in a few seconds. Even though the lighter M-16 ammo allowed each soldier to carry 20 or more magazines, in a heavy fight, especially in the jungle where inexperienced troops would empty a magazine at any sound, it was easy to run out of bullets. The World War II veterans knew this would happen. When the semi-automatic M-1 Garand replaced the single shot M-1903 bolt action rifle, it was noted that troops still equipped with the M-1903 used an average of eight rounds a day, while those with the M-1 used an average of 40 rounds a day. In Vietnam, most units in the field had troops using over a hundred rounds a day. In the 1980s, a new version of the M-16 was issued that replaced full automatic fire with burst fire of three rounds each time the trigger was pulled. Troops complained that this removed the ability to use full automatic fire when there was an emergency. This is true, but only if the troops are well trained and disciplined enough to use the automatic fire only when needed. But many of the troops in Vietnam were not well trained or disciplined. After that war, training did improve, but experienced infantrymen thought that automatic fire was too inaccurate, even when needed. After much experimentation, if was found that a rifle firing three round bursts when the trigger was pulled gave you more firepower without losing accuracy. If you need a lot of firepower, just keep pulling the trigger quickly.