Not only are these two rounds often interchangeable and able to be shot from the same gun, their history is entwined with each other. In fact, the 5.56 NATO round didn’t come about until the .223 was produced. In 1957, Fairchild Industries set about to create and new lightweight combat rifle. They were commissioned, along with several other groups of engineers, by the US Continental Army Command (CONARC) with a set of specific criteria that the gun and round had to meet. These parameters included:
· .22 Caliber
· Able to exceed supersonic speeds at 500 yards
· Rifle itself could only weigh 6 lbs.
· Able to penetrate both a steel helmet and .135” steel plate at 500 yards.
· Magazine capacity of at least 20 rounds.
These specifications came as a result of the 7.62 NATO round being deemed has having too much power to be carried by infantry. In May of 1957, Eugene Stoner shot the first AR 15 design to CONARC. Through further developments, the AR-15 was transformed into the M16 rifle that is still the standard rifle for most of the American Armed forces.
The difference between the two rounds? Pressure. The outside case dimensions are essentially the exact same for both rounds. However, with the military having different pressure testing procedures than the civilian institute of SAAMI, the two rounds often are loaded to different specifications. Simply put, the .223 to generally used by civilians while 5.56 NATO is often found in military surplus buys.