Prologue: This is not about the civilian legal military look alike rifles. Those rifles DO NOT possess fully automatic or burst fire capability. True assault rifles are not available to the public, outside of carefully licensed collectors, only the military and police possess them.
The U.S. Military has never had a comfortable relationship with the assault rifle. Deep seeded ideas of rifle marksmanship, fear of ammunition wastage, and the traditions of the rifleman going back to the Revolutionary War, have influenced U.S. rifle design to the present day.
Since 1945 assault rifle have established an enormous presence on the world’s battlefields. Armies and revolutionary groups have sought the advantages of the assault rifle.
The assault rifle was the result of a long evolutionary process which sought to provide usable firepower to the individual soldier.
Repeating rifles. The U.S. military establishment had an aversion to the repeating rifle starting in the Civil War. The Army ordnance establishment was very afraid that soldiers would waste precious ammunition in battle. This false fear was reinforced by a distrust of the new systems, such as the Henry, and Spencer rifles, which achieved substantially higher rates of fire that the rifled musket or early breach loading carbines. Early repeating rifles were essentially forced on the Army ordnance establishment during the war. This same establishment quickly returned to single shot rifles and carbines, shortly after the end of the war. Although these post war rifles and carbines used metallic cartridges, their rates of fire were well below the rates achieved by repeating rifles.
The Army steadfastly clung to the single shot rifle until the 1890s. The single shot black powder rifles were still widely used by National Guard or volunteer soldiers in the Spanish American War. Smokeless powder high velocity cartridges facilitated the adoption of a magazine fed bolt action rifle. These rifles fired powerful cartridges which could engage enemy soldiers at ranges up to 700 yards. Interestingly, the magazine fed bolt action rifles adopted by the Army, the 1892 Krag and the later 1903 Springfield rifles, incorporated a magazine cutoff which caused the rifle to function as a single shot, thus saving the cartridges in the magazine for “emergencies”. The phobia that soldiers would waste ammunition would rear its ugly head again and again.