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

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.

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

Dennis wrote:
October 18, 2013

I have a Green Label Colt AR15 HBAR Sporter. The owners manual states suitable ammunition is 223 Remington(5.56mm). Does this mean the chamber is designed for the 5.56mm ammo?

Richard Bernstein wrote:
March 13, 2013

I have read that when a .223 rifle is chambered just for that cartridge, it is much more accurate then one chambered for both .223/5.56 .. I believe that the Ruger ranch rifle, the target model is chambered only for the .223.

Gordon wrote:
March 13, 2013

As for reloading I have read several articals several from AR. There is a difference in civilian brass and military brass as military brass has a thicker wall and their for less volume. Having less volume will translate to greater pressure. If you load military brass it is wise to back off about 10% and work your load back up four your rifle. And remember, the advertised velocities are normally inflated. Also the condition of your chamber and bore will effect velocity. It is never a good idea to try to match any ones published velosities

Martin DePew OLS wrote:
March 11, 2013

Of all the articles that I haave read, they have for the majority indicted that the 5.56 and .223 ARE NOT IDENTICAL. However, I believe that those who have the special equipment for measuring the external cases should be able to tell the differences. I caution on the side of saying that they are similar but they are definately different.

Norman Hatfield wrote:
March 06, 2013

Well, I beg to differ. You state "Externally, the two cartridge cases are identical." Then you go on to say that 5.56 ammo is factory loaded to 60,000 psi while .223 is factory loaded to 55,000 psi. IF .223 ammo WAS loaded/reloaded to 60,000 psi would it be unsafe? The reason for the confusion is as you have stated, the difference between the normal chamber sizes beween 'military' and 'civilian' firearms. The military chambers are larger because they want to make sure their firearms can 'take a licking and keep on ticking'! Makers of civilian weapons assume the we're not going to be in an 8 hour firefight without being able to stop and clean them. So, they choose to chamber them for accuracy instead. ALL 'match' chambers are tighter dimensionally than 'standard' chambers. In fact you can get 'AR' rifles with TIGHTER than 'normal' .223 chambers. Doe that then make them unsuitable or dangerous to fire a box of plain jane Federal or Remington .223 in them? So, no. There is NO DIFFERENCE between 5.56 and .223 Remington ammmo. Would you be afraid to fire 7.62 Nato ammo in your rifle because it wasn't marked .308 Winchester? It's the 'same difference'.

Dennis Amnott wrote:
March 06, 2013

Before using my new Sig Sauer 516 we used a bullet comparator to check for proper leade and chambering. All worked well but it a little more accurate with the .223 as we have .010" less leade with commercial ammo than with the Lake City Green Tip. Of course maybe it was the shooter that day.

David wrote:
March 05, 2013

I have a Bushmaster M4A3 and it accepts both 5.56 NATO and .223. It's sweet as pie with both, but I've found that accuracy wise the .223 seems to work a bit better. Every individual weapon has it's own personality, but that's what works best in mine.

Greg wrote:
March 05, 2013

I remember there were problems with ammo and accuracy when we switched from the A1 to the A2. The ammo we received on the range was green tipped and designed for the A2 so the shots from the A1 were wild. Hand to cancel qualification that week until the ammo could be sorted out.

Bobby wrote:
March 05, 2013

I've never had a problem cycling 5.56 out of 223 or 223 out of a 5.56

Todd wrote:
March 05, 2013

I have a Stag Arms Ar and I have used both through it and haven't had any problems with either.

Tim wrote:
March 05, 2013

I have not had any problems cycling .223 through my 5.56 Del Tons either

Joel wrote:
March 04, 2013

Thanks. One topic which seems not covered is reloading to 5.56 levels. The NRA should consider covering this one. Can it be done safely, how, what are the benefits, under what conditions?