.223 Rem. vs. 5.56x45 mm NATO: Is It Safe?

by
posted on March 4, 2013
** When you buy products through the links on our site, we may earn a commission that supports NRA's mission to protect, preserve and defend the Second Amendment. **
keefe2015_fs.jpg

I have received a slew of questions—many from first time AR-type rifle buyers—about the .223 Rem. and the 5.56x45 mm NATO cartridges. Can I shoot 5.56x45 mm NATO in my .223 and vice-versa? Are these the same cartridge?

Externally, the two cartridge cases are identical. The main differences are that 5.56x45 mm NATO operates at a higher chamber pressure (about 60,000 p.s.i. versus 55,000 p.s.i. on the .223 Rem.) and the 5.56’s chamber is slightly larger than that of the .223 Rem. Also, the throat or leade is longer in the 5.56x45 mm chamber. What does this mean? You should not shoot 5.56x45 mm NATO out of a rifle that is chambered in .223 Rem. And be aware that some .223 Rem. ammunition will not reliably cycle through some AR-style .223 Rem. rifles, but it usually does. As a matter of fact, I have not encountered any difficulty with current .223 Rem. loads cycling through a 5.56 mm AR-style rifle.

Generally, most AR-style rifles are intended to accept 5.56x45 mm NATO and are marked “5.56,” while some are marked “5.56/223.” In the case of an AR-style rifle marked “223”, check with the maker as it may actually have a 5.56 chamber. American Rifleman covered this topic at length with an article by Field Editor Bryce M. Towsley.

Latest

Mossberg 940 Pro New Models 01
Mossberg 940 Pro New Models 01

Mossberg Introduces SPX & Professional Models To 940 Pro Tactical Line

Mossberg has expanded its popular 940 Pro Tactical shotgun line with SPX and Professional models, with each adding a slate of upgraded components and accessories designed to enhance the shotgun's capabilities.

Preview: Streamlight Sidewinder X

This high-output, multi-fuel, military-style light with a tilting head is perfect for hunters and home defenders looking for more than just a handheld light...

The Armed Citizen® July 4, 2025

Read today's "The Armed Citizen" entry for real stories of law-abiding citizens, past and present, who used their firearms to save lives.

A Guide To Double-Stack 1911s

The 2011 brings double-stack capacity and modular upgrades to the 1911 platform, but not every wide-body 1911 out there earns that name.

Legislators Enjoy Afternoon Of Clays With Benelli USA, Among Others

A bipartisan group of Maryland state legislators met with firearm industry manufacturers and representatives for a day of clays shooting while also attending discussions on the positive economic impacts of the shooting sports.

2025 Ammunition Product Of The Year: Winchester .21 Sharp

For continuing to innovate, particularly in a corner of the ammo world that has seen very little recently, we named the Winchester .21 Sharp as our Ammunition Product Of The Year.

Interests



Get the best of American Rifleman delivered to your inbox.