The M16 was originally intended to fire the 7.62mm NATO round

Today, the M16 rifle and M4 carbine are ubiquitous among American troops. These lightweight rifles, which both fire the 5.56mm NATO round, have been around for decades and are mainstays. The civilian version, the AR-15, is owned by at least five million Americans. But the troops hauling it around almost got a similar rifle in the 1950s that fired the 7.62mm NATO round.

It's not the first classic rifle to be designed to fire one cartridge and enter service firing another. The M1 Garand, when it was first designed, was chambered for the .276 Pedersen round. The reason that round never caught on? The Army had tons of .30-06 ammo in storage, and so the legendary semi-auto rifle was adapted to work with what was available.

The story is much different for the M16. Eugene Stoner's original design was called the AR-10 (the "AR" stood for "Armalite Rifle" — Armalite was to manufacture the weapon). This early design was a 7.62mm NATO rifle with a 20-round box magazine.

According to the National Rifle Association Museum, this rifle went head to-head with the FN FAL and the T44 to replace the M1 Garand. The T44 won out and was introduced to service as the M14. This doesn't mean the AR-10 was a complete loss, however. Sudan and Portugal both bought the AR-10 for their troops to use and, from there, the rifle trickled into a few other places as well.

Armalite, though, wasn't ready to give up on getting that juicy U.S. military contract, so they began work on scaling down the AR-10 for the 5.56mm cartridge. The Army tried the resulting rifle, the AR-15, out in 1958 and liked what the saw, pointing to a need for a lightweight infantry rifle. It was the Air Force, though, that was the first service to buy the rifle, calling it the M16, which serves American troops today.

Despite the immense popularity of the M16, the AR-10 never faded completely into obscurity. During the War on Terror, operation experience called for a heavier-hitting rifle with longer range. In a way, the AR-10 made a comeback — this time as a designated marksman rifle in the form of modified systems, like the M110 Semi-Automatic Sniper System and Mk 11 rifle.

Over the years, the AR-10 has thrived as a semi-auto-only weapon, available on the civilian market, produced by companies like Rock River Arms and DPMS. In a sense, the AR-10 has come full circle.