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5.56 Mil-Spec Chamber vs .223 Remington Chamber In AR-15


5.56 Mil-Spec Chamber vs .223 Remington Chamber In AR-15

5.56 vs 223 ammo. Are you confused?
Don’t feel bad, AR 15 ammo type is confusing at first.
Most gun dealers really don’t explain the 5.56 vs 223 dilemmas.
And it is a dilemma depending on what your gun is calibrated for!
In this article, I will explain the difference between 5.56 and 223 ammo in regards to your AR 15.
Can your AR 15 fire 5.56 ammo? I’m going to explain all of that!

.223 Ammo vs. 5.56 Ammo Comprehensive Guide

There are a great many factors to consider when looking at .223 ammo vs. 5.56 ammo for your AR-15. It mostly comes down to which one safely works inside your gun. And that comes down to the chamber that your gun comes with.

.223 Remington Chamber Is Most Common

Before the M&P Sport 2 became one of the best selling AR-15’s on the market, the .223 Remington Chamber was the most popular AR-15 chamber version. It still probably is when you consider the greater metrics, however, the increased sales in the M&P Sport 2 (its the top-selling budget AR-15 in the country now) are giving such an idea a run for its money. The AR-15, generally, has a chrome lined barrel and chamber.

A .223 Remington chamber is made to Sporting Arms and Ammunition Manufacturers Institute specifications. This results in a tighter chamber space when compared to a Mil-Spec 5.56 chamber. This means that firing 5.56 through the tighter chamber space could harm the chamber. This means you can only fire .223 through the barrel.

5.56 Ammunition Does Not Match .223 Remington Chamber

Let me say this loud and clear, your .223 Remington chamber will not support 5.56 ammunition, no matter what you’ve heard. There just isn’t enough room to safely fire this more robust, more explosive ammunition through it. Sure, it may fire seemingly fine, but given time, you’ll ruin your gun. A 5.56 emanates more heat, which creates more pressure inside the chamber; more than the 2.23 Remington barrel is made to handle. It isn’t the size of the bullet, its the release of pressure, which matters. And this is often confusing to those who hold a 5.56 and .223 round next to one another.

So what happens from the continued firing of 5.56 rounds through a .223 chamber? You may compromise the primer, which will get popped back into the receiver due to taking on more pressure than its built to handle. You may start finding that your AR-15 doesn’t eject ammo casings anymore. You may end up with breaks inside of your AR-15’s upper-receiver.

Often times, new AR-15 owners with .223 Remington chambers think that because they fired the 5.56 through the gun, the gun is fine for the 5.56 ammunition. These concepts aren’t one in the same. The ammo may fire, but the .223 barrel and chamber don’t support it; therefore, you will end up ruining your gun.

.223 Remington Chamber and 77 Grain

If your AR-15 chamber is .223, you shouldn’t fire a heavier 77 grain round through it. This may not seem like a big deal at first, however, 77-grain rounds are some of the most accurate rounds. This lack of .223 versatility could, down the line, become an issue for you.

77 grain .223 rounds tend to protrude further inside the barrel. If you try to eject a loaded 77 grain round from your .223 barrel, it may end up stuck. And then you will need to remove it using some elbow grease and a long rod.

You could solve this issue using a Match Chamber, but that’s another conversation altogether.

5.56 Mil-Spec Chambers vs. .223 SAAMI spec chambers

These are versatile and chrome lined chambers and is a tad larger than a .223 Remington chamber. As discussed in the earlier sections, that small amount of space can support the additional pressure from the more explosive 5.56 round. That’s what it is built for, so it stands to reason that it could support a slightly less explosive, pressure-inducing .223 round. Additionally, it could support a .223 77 grain round.

This means that you 5.56 Mil-Spec chamber is much more versatile than your .223 SAAMI spec chamber.

Use .223 and 5.56 ammo

The first benefit to a 5.56 Mil-Spec chamber is that it will safely accommodate both .223 and 5.56 ammo. You will not have to worry about damaging your barrel, chamber, or AR-15 upper receiver. For a prepper, this means more opportunity for ammo. The greater your ammo options are, the more likely you’d be able to find that ammo during SHTF. What if doomsday happened via a massive EMP attack and the power grid shut down for months, even years. What if martial law took effect and .223 rounds were confiscated and the only ammo you could find was ammo stolen from military agencies? Yeah, I know, it sounds like a bad episode of Walking Dead, but truth be told, this is a prepper website and we do focus on prepping for worse case scenarios. Ammo versatility plays perfectly with prepper logic.

Some professional shooters claim that firing .223 ammunition through a 5.56 Mil-Spec chamber results in less accuracy, though this should be nominal. Additionally, it is extremely safe to do so, which is the most important part. Putting .223 rounds through a 5.56 Mil-Spec chamber certainly isn’t preferred by snipers, but it is a great benefit for preppers and hunters, and for self-defense.

No common shooter would ever notice any substantial accuracy differences. This is all benefit.

Use 77 Grain in 5.56 Mil-Spec Chamber

As discussed prior, the 77 grain round juts out too far in the .223 barrel, which leads to complications in live round ejections, among other things. The 5.56 Mil-Spec chamber has a little extra length to its free bore area. Because of this, you should have no live round ejection issues to speak of. This also plays into the “ammo versatility” prepper issue we discussed above, though it’s much more negligible in terms of its gravity.


The 5.56 Mil-Spec chamber AR-15 offers much more ammo versatility than the .223 Remington chamber. This also means an enhanced ability at firing 77 grain, heavier ammo through the barrel. A .223 Remington chamber should never fire 5.56 ammo through it and it may experience difficulties ejecting live 77 grain .223 ammo from it.

The M&P Sport 2 is a budget AR-15 that has a 5.56 Mil-Spec chamber. You can read my M&P Sport 2 review for more information on it. At the end of the day, .223 ammo is readily available and very affordable, as well as accurate. I wouldn’t feel slighted if you purchased an AR-15 with a .223 Remington chamber; however, I’d look at such things for future purchases. There is nothing wrong with added versatility.


Author: Jim Satney

PrepForThat’s Editor and lead writer for political, survival, and weather categories.

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