The M-16 style rifle has become ubiquitous in imagery of the US military but it wasn't always that way.

As the US and its allies set about looking for a more reliable, accurate, and lighter general issue weapon, America's adoption of the 5.56mm round and the service rifle that fires it came about.

In the early 1950s, the North Atlantic Treaty Organization set about looking for a single rifle cartridge that could be adopted throughout the alliance, making it easier and cheaper to procure and distribute ammunition force-wide, and adding a bit of interoperability to the widely diverse military.

Despite some concerns about recoil, the 7.62 x 51mm NATO round was adopted by the United States in 1954.

Bullet cartridge shell 9mm 7.62mm 5.56mm
Left to right: 7.62 mm NATO, 5.56 mm NATO, and 9 mm NATO cartridges.
Fourdee/English Wikipedia

The 7.62 x 51mm round is more similar to the.308 than the 7.62 x 39mm round used in Soviet AKs, but it isn't long before even the 7.62's biggest round is used.

These rounds were powerful and accurate, but they were heavy, expensive and created a lot of recoil as compared to the service rifles of the modern era.

Early development of a new round and rifle platform began in 1957. Despite the smaller projectile, US specifications required that it maintain supersonic speed beyond 500 yards and be able to penetrate a standard-issue helmet at that same distance.

Plans for new bullets and a new rifle were quickly drawn up after the US military asked for something other than what was already available.

Army National Guard Alaska 5.56mm ammunition ammo bullet
Strips of 5.56 mm ammunition collected by Alaska Army National Guard soldiers during M249 light machine gun training at Joint Base Elmendorf-Richardson, Alaska, March 3, 2018.
US Air Force/Alejandro Peña

The.222 round was converted into the.222 Special to give the US military a bigger punch. The new round was designed to meet the performance standards set by the Pentagon. The.222 Special was better suited for magazine feeding because it was in a longer case.

The.222 Special was one of the two calibers of rounds your rifle can shoot.

The new round was called the 5.56 x 45mm NATO and was based off of the.223 caliber design. This new round exceeded the Defense Department's requirements for muzzle velocity and range, and fired exceedingly well.

Many experts believed that the new rifle was superior to the M14 despite the fact that there were still some issues that needed to be worked out.

Marines 7.62mm ammunition ammo kevlar helmet bullet
US Marines stage a belt of 7.62 ammunition inside of a kevlar helmet during a squad live-fire range aboard Babadag Training Area, Romania, June 6, 2018.
US Marine Corps/Cpl. Alexander Sturdivant

The new AR-15 was capable of firing.223 rounds and later, the 5.56mm rounds, which was scaled down from the 7.62 chambered version. It was able to choose between semi-automatic and fully-automatic modes of fire and 20 round magazine capacity, all of which were required for a new service rifle.

It was a match made in heaven, and branches began buying the rifles in the 1960s. The standard for the alliance was not adopted until 1980.

The decision to switch from 7.62 x 51mm to 5.56 x 45mm was simple.

The smaller rounds allowed troops to carry more weapons into the battle. They made it easier to level the weapon back onto the target between rounds. The tests showed that troops with smaller rounds were more effective at engaging targets.

In Marine Corps rifle teams, the goal is to locate, close with, and destroy the enemy, and the 5.56mm NATO round made them better at that.

The article has been updated.