Why Did the U.S. Military Choose the 5.56 NATO Cartridge?

The U.S. military adopted the 5.56mm NATO cartridge in 1963, during the Vietnam War. Ever since, there’s been contention about whether or not it was a good change. The dissent over the switch to 5.56mm leads us to question why the U.S. would pick up the 5.56 NATO round at all, if it’s not clearly a better option.

But, most of the contention surrounding the 5.56mm NATO round is based on incomplete consideration. The military must account for things beyond terminal ballistics when it comes to choosing a service round.

As such, it was a combination of factors that spurred the U.S. military to adopt the 5.56mm NATO cartridge. Here’s what drove their decision.

Round Count

During the Vietnam era, most nations opposing the U.S. and other western nations were issuing AK-47 or AKM rifles, which fire the 7.62×39 round.

The 5.56mm cartridge is both smaller and lighter than the 7.62mm round. So, U.S. troops could carry more rounds—often with less total weight—than enemy soldiers.

Strategically, this gave U.S. units an advantage because they could outlast and outmaneuver enemy forces. But, the benefits of using a smaller round also extended beyond strategic and tactical advantages.

It was also easier to transport 5.56mm ammunition. It was less resource intensive to resupply units in the field with 5.56mm ammunition than it was to deliver 7.62mm rounds. And, units needed to be resupplied less frequently, because they could carry more ammunition with them.

So, switching to a smaller round made the U.S. military machine more efficient from front to back.


Two of the defining characteristics of the 5.56mm round are the high velocity and flat trajectory. The recoil is also exceptionally light. This improved overall U.S. military combat marksmanship, since troops could shoot faster and use a more direct point of aim.

Early on, U.S. forces were issued fully automatic rifles because the high command believed that the 5.56 was controllable enough that full-auto fire would still deliver acceptable accuracy.

However, these 5.56 NATO firearms dumped ammunition incredibly fast, and the accuracy was much worse than anticipated during full-auto fire. But, soldiers exhibited improved marksmanship in semi-automatic fire. So, when the U.S. upgraded to the M16A2, they removed the fully automatic function to maintain an overall combat accuracy standard.

No matter which round you’re shooting, shot placement is king. So, military officials prioritized an increase in marksmanship among the military general population over terminal ballistics.

Terminal Ballistics

This is where most people make their case against the 5.56mm, citing that the 7.62mm has better wounding capacity and stopping power.

However, the 5.56mm round delivers plenty of lethality in battlefield applications. Most complaints about the effectiveness of the 5.56mm round stem from the shift to the M855 round.

The U.S. military started using the M855 round because they wanted better steel armor penetration. The M855 featured a steel penetrator in the bullet and a slightly lower muzzle velocity. It was a bit of a compromise between terminal ballistics and penetration. Consequently, the M855 didn’t tumble and fragment in soft tissue as well as the original M193 round.

Additionally, both the M855 and M193 rounds were designed to be fired from at least a 20 inch barrel. When U.S. forces started transitioning to carbines with 14.5 inch barrels, the 5.56mm round often didn’t perform as intended, because it wasn’t being used as intended.

The M193 round didn’t suffer much from a shorter barrel. However, with M855, the lower muzzle velocity significantly reduced the round’s tendency to tumble and fragment. With a carbine, shots to the extremities were less catastrophic than from a full-length barrel. So, hits to the torso or head were usually required to stop enemies.

But, M855 is still impressively lethal. Even so, the military eventually upgraded to the M855A1 round, which improved both the terminal ballistics and armor penetrating capabilities of the 5.56mm round.

In the end, the 5.56mm round offers plenty of battlefield effectiveness. Enough so that the higher echelons of the U.S. military felt comfortable—and still feel comfortable—using it as a standard service round.

Will it Ever Change?

Although the 5.56mm round is still the reigning champion among western militaries, many NATO forces have acknowledged that there is most likely a round that offers better wounding capacity without sacrificing the logistical benefits and marksmanship advantage that the 5.56mm round offers.

Both the U.S. Army and the Marine Corps have voiced an intent to develop a better round by 2023. Current iterations are based on the 6.5 Creedmoor cartridge.

However, logistical and funding hurdles will likely prolong the service of the 5.56mm beyond the development deadline. And, even if the U.S. military begins adopting a new round, the 5.56 NATO cartridge will likely be in use on a worldwide scale for decades to come.

Jay Chambers – Pro Free Speech Writer, Disaster Survivalist, Business Owner. Believes in Resiliency and Self Sufficiency in an Increasingly Unpredictable World.

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