The M16A4 began as the M16 Rifle. The M16 rifle was created in response the US Air Force’s need in the 1950’s for a lightweight, small-form “survival rifle” for self-defense in the event of a hostile landing. The order went out to Armalite Company, which had recently merged with Fairchild Engine and Airplane Corporation. A designer at Armalite named Eugene Stoner soon developed the 5.56mm (.223) cartridge and reworked it until he had 55grain bullet traveling at 3250fps and an effective range of about 500yds. With a reliable cartridge in place, Stoner began work on the rifle itself and soon came out with the AR-15 rifle. The AR-15 was a drastically different rifle in both design and function from anything that had been used by the US Air Force before. The AR-15 employed a direct gas system of operation instead of the conventional piston operation and made extensive use of light alloy and precision casting to reduce weight. The rifle had a straight line design instead of the traditional “dropped-butt” of conventional rifles and the sights were permanently attached to the carry handle. The radical new design, along with the rifle’s plastic furniture, led the US Army and Air Force brass to dub the rifle “cheap and nasty”. Nonetheless, in 1962 the US Air Force bought a number of AR-15s for use by guards at Air Force bases. Then, in 1963, the USAF sent a detachment armed with AR-15s to Vietnam and were surprised to find that the South Vietnamese Army felt the new rifles would be perfect for the smaller stature soldiers that made up the majority of their ranks. The South Vietnamese army soon bought AR-15s and reported favorably as did the US Special Forces serving in Vietnam at that time. Eventually, the AR-15 was taken into regular US Army service as the M16 Rifle.
You will learn the rest in boot camp.