How Bad Is It … to Mix Up 5.56 and .223 Ammo?

Are 5.56 and .223 the same? What happens if you get them mixed up?

by posted on August 23, 2023
Deering 556 Vs 223

The most popular rifle in the U.S., the AR-15, is most commonly chambered in 5.56 NATO, but you’ll hear many people refer to it as .223 Rem. This causes a lot of confusion, as the two cartridges are extremely similar, although not quite interchangeable. They fire the same caliber projectile, and the cartridges’ external dimensions are identical. But it’s what’s inside the case and inside the guns themselves that makes the difference.

The short answer is that you can shoot .223 Rem. ammo out of a gun chambered for 5.56 NATO, but you cannot safely shoot 5.56 ammo out of a gun that’s only marked for .223. How bad is it to mix the two up? One way is no big deal; the other is a safety hazard. Why?

There are two main differences between the cartridges. The first is that the 5.56 has a bit more powder and thus a slightly higher pressure level than the .223—about 58,000 psi vs 55,000 psi. Not huge, but it matters.

The second difference is that rifles chambered for 5.56 have a slightly longer “throat,” and 5.56 ammo is designed to work with that length. When you fire a 5.56 cartridge out of a .223 rifle with a slightly shorter throat (the part of the chamber that’s in front of the case), it generates pressure of 65,000 psi or more. The .223 gun is just not designed for that amount of pressure, and it can cause everything from reliability problems to blown primers to damage to the firearm and the shooter.

Conversely, shooting a .223 cartridge out of a 5.56 rifle is safe, but because of that difference in throat length, you might not achieve the velocity and performance/accuracy you’re looking for. It might not even properly cycle the action of a semi-automatic 5.56 rifle, particularly with a barrel shorter than 14.5 inches (which most of us probably aren’t shooting, as it’s below the legal minimum barrel length for civilian firearms).

Bottom line: Never fire a 5.56 NATO cartridge out of a gun that is marked .223 Rem. — it’s not safe. It is safe to fire a .223 Rem. cartridge out of a gun marked 5.56 NATO, but you might suffer some velocity and accuracy loses. As always, if you have any doubt, just don’t load a firearm with any ammo other than what’s stamped on the gun.



NRA Women The Armed Citizen Shooting A Pistol
NRA Women The Armed Citizen Shooting A Pistol

The Armed Citizen® March 1, 2024

A brave mother and her three children hid in a closet after a knife wielding man broke into their home—but they weren't helpless.

Women for Gun Rights’ State Directors Graduate from 2024 Ambassador Academy

The Ambassador Academy provided a platform for this group of women to refine their Second Amendment advocacy skills.

Low-Recoil Defensive Pistols: 5 Sub-Caliber Cartridges

Here’s a high level view of what smaller-than-9 mm pistol rounds have to offer.

Why I Hunt

There are endless benefits to hunting, but here are four of the main reasons that drive people to the field.

New Guns 2024: Smith & Wesson M&P15 Sport III Rifle

The new Sport III comes equipped with enhancements to increase performance—while being offered at a lower price.

Walther Arms Sponsors 2024 NRA World Shooting Championship

This competition gathers the top shooters in the world at Camp Atterbury, Indiana, to compete in both pro and amateur divisions.


Women's Interests

Get the best of NRA Women delivered to your inbox.