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

by
posted on March 4, 2013
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

Rossi Brawler 01
Rossi Brawler 01

New For 2025: Rossi Brawler In 5.56 NATO & .300 Blackout

Building on the .45 Colt/.410 bore Brawler introduced in 2024, Rossi USA is expanding its line of break-action handguns to include chamberings in 5.56 NATO and .300 Blackout.

Preview: Winchester Supreme W20-60X 80 mm Spotting Scope

Sure, the skilled use of your firearm is vital to the success of any big-game hunting adventure, but you’ll first need to find your quarry, and Winchester’s new Supreme line of optics includes a pair of reasonably priced spotting scopes that can help put you on the trail of your target animal.

Gun Of The Week: Heritage Mfg. Roscoe

Welcome to another American Rifleman Gun Of The Week video, and this time, we’re checking out a revolver fit for all the private eye wannabes and film noir fans out there. This is the Roscoe from Heritage Manufacturing.

The Armed Citizen® May 30, 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.

Nazi Germany's Last-Ditch SMGs Of World War II

As the war in Western Europe began reaching its inevitable conclusion, German efforts to expedite arms production led to some of the most unusual submachine guns in its arsenal.

New Optics For 2025

For 2025, your American Rifleman staff engaged with the latest optic products available for the year, with a few of the most interesting noted here.

Interests



Get the best of American Rifleman delivered to your inbox.