Pet Peeve - A marine said " I want a 556 AR, Not a 223"

Simply put, do NOT fire 5.56 NATO in a .223 chambered firearm. The dimensions are NOT the same, specifically the leade dimensions.

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So a thought, which seems to make all this discussion “moot”. When you are acquiring an AR15 rifle why would anyone simply not get a 556 rated weapon? My Colts all being milspec are 556 capable. I see no advantage to consider purchasing a rifle ONLY rated for 223. The safest bet is to always acquire a “milspec” rifle. Our service members all use milspec!! What am I missing?

LONG story short: The .223 Remington and 5.56 NATO fire exactly the same caliber (diameter) bullet. The chamber between the .223 Remington and 5.56x45 NATO are identical EXCEPT for the throat leade dimension. The leade is essentially the clearance or distance to the rifling that the bullet has to make after the powder is ignited. In order to ensure reliability with various types of ammunition and conditions found on the battlefield, the 5.56 NATO chamber has a larger leade than the .223 Remington chamber. Also, as stated previously, 5.56 NATO is rated at a slightly higher pressure than .223 Remington. So, if you fire 5.56 NATO in the slightly tighter leade of the .223 Remington chamber, you run the risk of unsafe pressure spikes due to a combination of higher rated pressure, and the bullet not having enough of a “running start” to the rifling as the cartridge is ignited. Conversely, it is safe to fire .223 Remington in a 5.56 NATO chamber because there is no risk of pressure spikes (larger leade and lower rated pressure). The only downside in this scenario is that, due to the greater jump that the bullet has to make to the rifling in a 5.56 NATO chamber, inherent accuracy and shot precision versus a dedicated .223 Remington chamber will be reduced. The .223 Remington was originally meant to be a varmint hunting cartridge fired out of a bolt action rifle. In that application, tight chamber tolerances are vital to the high degree of accuracy required for varmint hunting. 5.56 NATO is a military cartridge where reliabilty in a wide range of rifles and conditions trumps accuracy potential. If nothing else, remember that you can safely fire BOTH .223 Remington and 5.56 NATO in a 5.56 NATO chamber. You can fire ONLY .223 Remington in a .223 Remington marked chamber.


@Smiddy You are correct. The 5.56 bullet does not measure .223, but neither does the .223. Both bullet designations measure .224 which would make them somewhere around 5.7mm actually.
The difference is not in the bullet but in the case, and true chambered 5.56 rifles are built just a bit differently.
Sure you’ll probably be okay firing 5.56 in a .223 chamber, but the differences and failures have been documented. If anyone wants to know for sure, do not base your decision on arguments in a forum. This is discussion, not necessarily factual. Go to the authority for the facts and then make your decision whether to try it or not. The term authority is based on the word author…the person who designed the thing or wrote the book. The true authority (not just a video or article by one who did not design and test the rounds) is the last word.
I’ve commented here not to dispute any of you. You can each be correct in your own mind. That is your decision, not mine. I’ve commented because I don’t believe any of us would want someone to be hurt or killed because they took one of our posted pieces of advice. I only use ammo designed and marked for a particular weapon. Frankly, all my AR’s that take a .224 diameter bullet are designed for multi caliber… both.223 and 5.56.
Everyone one should go to the authority and not give authority to us. I don’t want to be perceived as the authority. I don’t want that responsibility.


@Nathan Well spoken.

Oh, I love this video, @Joe35! Thanks for sharing it! Welcome to the Community.


This is why I chose the hybrid .223 /5.56 chamber designed by Bill Wylde.


Yes they are ALMOST identical in all aspects with the exception that a 556 AR can shoot both a 556 and 223 round, while a 223 AR can only shoot a 223 round. So if he was complaining about wanting a 556 AR there is some validity to his complaint if he wants to shoot 556 ammo rather than going with the 223 only.

But this reply seems redundant as everyone seems to be echoing the same info. Me personally I will only buy a 556 AR so that I have the option between to two rounds.


There is a concern with military surplus and civilian. Civilian ammo may generally be used in civilian firearms, and military surplus ammo can also be used in civilian firearms, but military surplus rifles, due to several factors such as brass thickness, headspace, chambering, rifling and pressures…. does not do as well with civilian ammunition, even if it has the same designations or dimensions.
.308 / 7.62x51 , the .308 has higher pressure and the thicker brass on the military ammunition to prevent damage on ejection, means the thinner brass of commercial rds may rupture or tear apart in the military surplus firearm.
the .223 has lower pressure than the 5.56x45. With the pressure tolerances, and other issues, a firearm designed to use .223 should perhaps not have 5.56 fired in it, not without testing it.

I’m reading both sides of this, but at the end of the day, I’m not going to shoot a round through my rifle if the manufacturer tells me not to. If I don’t trust the manufacturer to tell me what ammo works with my rifle, then I don’t trust that manufacturer.

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Colt is built for 5.56, even when stated .223. So only in a Colt .223 can one safely fire 5.56.