Can any damage occur to a Ar15 who has a 223 chamber -but has a 5.56 nato round fired in it?

It depends on the tolerances to which your rifle has been manufactured.  As a general rule though it is unwise to shoot 5.56 in your .223 chambered rifle.

Thanks- I'm looking at a good deal on a upper- but just found out it is for 223 only....Thats okay- I'll shoot my surplus shit in the AK....

I have fired untold thousands of 5.56 NATO ammo in Remington 700s & Mini Mark X Mausers with 223 Rem chambers without incident. I'm not going to tell you its safe to do so in an AR with a 223 chamber, but I will say that external case dimensions are identical & minor differences in pressures are vastly overblown IMO. Personnely I'd rather have my ARs chambered for 5.56 or the Wilde chamber myself. I also think that the tighter tolerences of a commercial 223 chamber might give a very slight theoretical edge to accuracy, but they are also vastly overblown IMO. I've seen some very accurate ARS with a 5.56 chamber, particularly with handloads crafted to fit their slightly larger chambers. Armalite used to have a very interesting discusion of this issue on their website, not sure if its still there or not. The bottom line of it was if you are experencing ammo problems in their ARs with 223 chambers, more than likely the ammo doesn't meet 223 SAMMI or 5.56 NATO specs. I fully agreee with that statement.  I feel the main thing for an owner of a 223 chambered AR to be aware of is to never use a handload devolped for a 5.56 chamber. M9

I just checked & Armalite still has a posting on this subject. I find it to be the most sensible posting on this conterversary of any AR manufacturer. Some of their positions are absurd. Here is a link:  http://www.armalite.com/library/techNotes/tnote45.htm

I would purchase a rifle in 5.56mm instead of .223.  

I know Bushmaster states that a 5.56mm round is loaded to produce higher velocity and CHAMBER pressures.  They further state you should not use 5.56mm ammunition in rifles that are not specifically chamber for them as it is considered by SAAMI to be an unsafe combination.

You can shoot .223 out of a 5.56mm but not recommended the other way around.

Hope that helps.

but I will say that external case dimensions are identical

Not so, www.ammo-oracle.com

I haven't seen any problems with decent ammo.

What type of .223is it, some such as .223 Wylde are designed to fire both.

Quoted: but I will say that external case dimensions are identical Not so, www.ammo-oracle.com

Have you actually read your link?  Because it says you're wrong:

Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

Though some people want to call these two distinctly different ctgs. I'd remind everyone that there is only one ctg drawing & only one reloading die. The ctg was invented by Remington at Armalites request. It was originaly called the 222 Remington special. At that time Remington had 2 different 222 Remington ctgs, The 222 Remington & the 222 Remington magnum. The original AR15 was chambered for 222 Remington. The Military changed the spec from desired dbl penetration of a GI helmet @ 300 yds to 500 yds ( Some say they thought that was an unobtainable goal & would eliminate the unwanted AR, as they were quite happy with the M14 at the time & were being forced against their will to test the AR).The 222 Remington was capable of that @ 300 but not @ 500. The 222 Remington magnum was capable of it @ 500 but was too long for the already existing AR magazine. So they split the difference between the two & came up with a 3rd 222 Remington ctg, the 222 Remington special which later became the 223 Remington in SAMMI specs, perhaps to avoid confusion, since they already had two 222 Remington ctgs & a 3rd would only increase confusion. At this time Remington would not have touched a Metric designation with a 10 foot pole. They had recently introduced the 6mm Remington & Winchester had introduced the very simular 243 Winchester. The Winchester rd though technically inferior too the 6mm Remington became a great success while the 6mm Remington floundered. Remington concluded probrably correctly that the American shooter didn't want no durn weird metric ctg. The US Army intially called this ctg the 222 Special also, but was shortly going through a metric conversion that was supposed to completly eliminate the English inch system in the US ( didn't happen despite the Govts best efforts), as a federal entity the US Army had no choice but too comply with orders to convert to the Metric system, it also has a limted amount of logic since most of our allies including the English have converted too the Metric system. So the official moniker the Army ended up giving the new ctg was 5.56 NATO. That doesn't mean they are complety different ctgs. M9

Quoted: Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

 I'm not totally disagreeing with that statement, but I think the pressure differences are more a matter of manufactering tolerance & USUALLY quite minimal. The official Army acceptance stds as to pressure for the 5.56 are 52,000 PSI Max average pressure, 58,000 PSI max allowable pressure for an individual ctg in a lot. Port pressure in the M16 is speced @ 15,000 psi + or - 2,000 psi. Proof ctgs are loaded to 70,000 psi. I'm not sure of SAMMI's specs. But be aware that some of SAMMI's specs are in CUP instead of psi. I do know that Federal & Winchester have both sold rejected lots of 5.56 NATO ammo repackaged as 223 Remington. Even though the ammo didn't meet NATO spec, it did meet SAMMI spec. I think that there is some overlap in the two specs. In other words some lots of ammo might satisfy both specs. And an individual lot might fail one or the other or both. Also be aware that Army acceptence stds don't cull a lot of ammo for insuficent pressure. They specify the powders that are acceptable & specify a certain spread of acceptable velocity with the specifed bullet. They would cull an individual lot for insufficent port pressure though. In other words the Army spec could care less about pressure as long as its not excessive & it gives the desired velocity with the desired powder & the desired port pressure. This is because lots of military ammo are loaded with bulk grade powder not canister grade powder. On any indivdual lot of ammo the desired bulk grade powder is used & since individual lots of bulk grade powder vary more than canister powder the charge is varied so as to obtain the desired velocity & port pressure.
M9

Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

 Be aware that in the above statement they are refering to external dimensions only as in a case drawing. Internal dimensions of cases vary from lot to lot & maufacturer to manufacturer. In general military cases are slightly thicker in an attempt to stall cook-off. Though that is generally true I have seen some lots of military cases that were lighter than some lots of commercial cases. The bottom line: A load that might be safe worked in one lot of cases could be unsafe in another. A general rule of thumb is to take a load worked up in commercial cases & reduce it 10% for military cases & then work up. This situation is not specific only to the 5.56/223, but applies to all reloaded ctgs. M9

M9- Thank you- very very interesting.

You are quite welcome. M9

Quoted: Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.  Be aware that in the above statement they are refering to external dimensions only as in a case drawing. Internal dimensions of cases vary from lot to lot & maufacturer to manufacturer. In general military cases are slightly thicker in an attempt to stall cook-off. Though that is generally true I have seen some lots of military cases that were lighter than some lots of commercial cases. The bottom line: A load that might be safe worked in one lot of cases could be unsafe in another. A general rule of thumb is to take a load worked up in commercial cases & reduce it 10% for military cases & then work up. This situation is not specific only to the 5.56/223, but applies to all reloaded ctgs. M9

The main reason for thicker cases in the 5.56 NATO compared to the .223 is the stress placed on them in automatic weapons.  When put through a machine gun that may have a very hot and dirty chamber along with a very high cyclic rate, a case that is sticking a bit in the chamber can have the head ripped off it during extraction which then leaves the weapon out of action with the separated case stuck in the chamber.  At a minimum, it will require a stuck case remover or barrel change to get the weapon back in action. If the gunner was firing full auto under those conditions, it's probably a bad time to clear a stuck case from the weapon or change a barrel.

The same applies in spades to the 7.62mm NATO which has substantially thicker walls than a .308 Win and a correspondingly smaller internal volume, which equates to higher pressures with a given powder charge.

Weapons design and related traditions also play a part as in the past it was not uncommon to have a machine gun suffer from set back and essentially operate with excessive headspace on a regular basis. The thicker case is again more tolerant of the additional stretch and is less likely to separate during firing or extraction.

With regard to ammo-oracle (tremendous wealth of information there, kudos to the author), though the external case dimension may be exactly the same, the chamber dimensions, are not, are they? Specifically, the lead and throate of the chamber are different.

With regard to chamber pressure... You're likely to find that a credible manufacturer tests all of their .223 chambers to specs that far exceed the cup of a 5.56 chamber.

Either way, what you said about just saving the surplus for your other rifle would make it a moot point.

Quoted: With regard to ammo-oracle (tremendous wealth of information there, kudos to the author), though the external case dimension may be exactly the same, the chamber dimensions, are not, are they? Specifically, the lead and throate of the chamber are different. With regard to chamber pressure... You're likely to find that a credible manufacturer tests all of their .223 chambers to specs that far exceed the cup of a 5.56 chamber. Either way, what you said about just saving the surplus for your other rifle would make it a moot point.

 Yes the chambers are different. Military chambers are looser & have more leade. They will both accept either ctg. M9

My bro just paid $12 for a box of SS109.  Olympic Ammunition?  The brass looked very polished and the green tips were bright too.  Legit?  It looked the part but I'm not sure.

Stuck cases, blown primers, torn case rims...

Quoted: Can any damage occur to a Ar15 who has a 223 chamber -but has a 5.56 nato round fired in it?

"A", as in one round?

I think you'd be okay.

I just ordered a RRA Elite CAR A4 and the site states that it's chambered for .223/5.56.  From what you guys know about RRA, is it safe to shoot either in their barrels?  I ordered it with the 1:9 chrome lined barrel.

Quoted: My bro just paid $12 for a box of SS109.  Olympic Ammunition?  The brass looked very polished and the green tips were bright too.  Legit?  It looked the part but I'm not sure.

Highjack?

Quoted: I just ordered a RRA Elite CAR A4 and the site states that it's chambered for .223/5.56.  From what you guys know about RRA, is it safe to shoot either in their barrels?  I ordered it with the 1:9 chrome lined barrel.

My understanding it is safe to fire both.  My RRA tactical entry has 5.56 stamped below the RRA logo on the lower receiver.  Yours probably does too.