Throwback Thursday: AR-15 .223 vs Mil-Spec 5.56mm Chambers

By Jerry Kraus published on in Firearms

What is the biggest difference between AR-15 .223 and Mil-Spec 5.56mm chambers? Most people do not know; they say, “I think you can shoot both kinds of ammo through either one, right?”

Then, they buy an AR-15 and start to get more educated. Frequently, they later have regrets they did not get the AR with a chamber they wanted because they did not know what to ask.

We are going to cover the major options in AR-15 chambers, including Mil-Spec chambers and the benefits of each. I am going to limit this to .223 Rem. and 5.56mm chambers and will address ARs with other calibers, such as .308 (7.62×51 NATO), in the future.

There are three types of chambers in the M16/M4/AR-15/MSR family of rifles, but most people think there are only two.

  • The first is the Mil-Spec 5.56mm chamber, which is used in the M16 and M4.
  • The second is the .223 chamber, the most common chamber in AR-15 rifles, although you can get AR-15s with a 5.56mm chamber (why you would want that is very interesting and I go into that in detail below).
  • The third is a .223 Match chamber, which is used in AR-15s by serious competitive AR-15 shooters.

.223 Rem Chamber

First, let us cover the most common AR-15 chamber, the .223 Remington chamber, commonly called .223 Rem. or just .223. Most AR-15s also come with chrome-lined chambers and barrels. It does not make sense in the manufacturing process to only have the bore (barrel) or only the chamber chrome lined, so if you read a spec sheet that says an AR has a “chrome-lined barrel,” you safely may assume the chamber is chrome lined as well.

.223 chambers are made to SAAMI (Sporting Arms and Ammunition Manufacturers Institute) specs, not Mil-Spec, so the chambers are slightly tighter and smaller than Mil-Spec 5.56 chambers. Normally, that is not a problem since the most plentiful ammo available to civilians is .223 and not 5.56mm. But many people buy ARs with .223 chambers because they do not know any better, and then they find out there are drawbacks to ARs with .223 chambers.

Drawback #1: The Myth of Using 5.56 mm Ammo in a .223

The first drawback to .223-chambered ARs is the myth that you can shoot 5.56mm Mil-Spec ammo through it. Manufacturers that print the two calibers on rifles and in rifle manuals synonymously further perpetuate that myth.

.223 Rem vs. 5.56mm Nato

From the outside, both cartridges look the same. However, looks are deceiving.

You can shoot 5.56 through your .223 chambered AR-15—but you may regret it.

Since 5.56mm Mil-Spec ammo is loaded hotter, it has higher chamber pressure. Built to SAAMI specs, not Mil-Spec, the .223 chamber is ever so slightly smaller than a 5.56 Mil-Spec chamber. So when you shoot 5.56 in a .223 chamber, the case cannot expand as much as it would in a 5.56 chamber.

Therefore, a couple of things happen with varying frequency. The most common is that you will blow primers; that means you will have the primer blow back into the receiver, which decreases reliability as it rattles around in your receiver or on top of your magazine.

You also will experience an increase in failures to eject the spent cases because the case has expanded so much from the hotter load in the smaller chamber, and you may not get the case out of the chamber without putting a rod down the barrel. Shooting Mil-Spec ammo through a .223 chamber also may crack your upper receiver; this is less common, but still happens, and is potentially dangerous to the shooter and nearby people.

So you can shoot 5.56 through a .223 chamber, but it is highly inadvisable.

Drawback #2: Heavy Bullets

The second big drawback to a .223 chamber is shooting heavier ammo—77 grains and above. This is the preferred bullet weight for national match shooters and snipers. The problem is that those rounds are slightly longer than lighter AR ammo, so the projectile is sticking slightly farther down the barrel when you chamber the round.

The problem becomes very obvious when you try to eject the heavier bullet-weight round from the chamber without firing it. This happens because the heavier projectile is slightly longer. On occasion, the rifling grooves may grab it when you try to eject it. The result is that you pull your charging handle back and the case comes off the bullet, spilling unspent powder on the receiver (and your magazine if you did not remove it first). As it ejects, you are left with a projectile in the barrel, and you will need a cleaning rod to knock it out. Then you will have a mess, and it is not fun—especially when you are on the firing line at Camp Perry competing for the national championships.

Match Chambers

This brings us to the .223 Match chamber. Most .223 Match chambers are not chrome lined. The biggest difference in .223 Match chambers is that the rifling does not begin as quickly, so you do not have the problems referenced above with the case coming off the projectile if you try to eject a live round from the chamber. This is the preferred chambering for serious competitive shooters who like to compete at the national level, such as at the NRA National High-Power Long Range matches and CMP (Civilian Marksmanship Program) matches at Camp Perry, Ohio, each summer.

Mil-Spec Chambers

Then there are Mil-Spec 5.56mm chambers. These are always chrome lined in the M16/M4s for the military and typically are for their semi-auto AR-15 brothers. The 5.56mm Mil-Spec chamber is slightly larger than a .223 SAAMI spec chamber because the Mil-Spec ammo is loaded hotter and has higher chamber pressures.

Benefit #1: Use Both .223 and 5.56 Ammo

So the supreme benefit of 5.56 chambers is that you can shoot .223 ammo and 5.56 out of a 5.56 chamber without reliability or safety concerns. That gives you the flexibility to take advantage of the great military surplus ammo bargains when they are available.

The downside is that, at greater distances, some shooters think they will see a decrease in accuracy shooting .223 ammo though a 5.56 chamber because the chamber is ever so slightly larger.

I think that is arguable. I know what you are thinking: “How much decrease in accuracy?” and “At what distances does it make a difference?”

I believe that 95% of shooters will not see a measurable difference, except at extreme distances for which they may not have the training to shoot effectively anyway. Remember, when it comes to shooting, most of the time “It’s the Indian, not the arrow.” Remember, I am talking about .223 ammo through a 5.56 chamber only—not 5.56 through a 5.56 chamber.

Benefit #2: The 5.56mm Chamber Has a Slightly Longer Throat

The second benefit of a 5.56mm chamber is that it also has a slightly longer throat/free bore area. In simpler English, that means that there is more space between the projectile and the rifling. Remember when I explained what happens when you try to eject a live round when it is 77 grains or above from a .223 chamber (not a .223 Match chamber)? Well that does not happen with 5.56mm chambers because of the longer throat.

Benefit #3: Availability of Ammo

The third benefit of having a 5.56mm chamber on your AR is a little paranoid, but not unfounded, although I pray it never happens. Some of my prepper fans out there believe there may be martial law one day in America, which would include an attempted disarmament of Americans.5.56 NATO Ammunition

That is what Hitler did, so it is not unimaginable.

In any case, nobody can argue that, in an extended time under martial law, you might only be able to get ammo by stealing it off the back of a Humvee—if you do not get shot trying to in the first place. I would want an AR-15 with a 5.56mm chamber so you can shoot military ammo through it without added potential reliability problems, and remember, you will still be able to shoot .223 ammo as well.

There is a way to ream out a .223 chamber and make it 5.56mm. I have heard that it is easy, although I have never done it. You might be able to find the reamer, but if I wanted that, I would have it done by a reputable gunsmith.

What should I buy?

So now you may be thinking, “This is really confusing; just tell me what I should get.”

So if I could only afford one AR, I would get one with a 5.56mm chamber.

I also would not trust the labeling on the spec sheet on the manufacturer’s website or even in the owner’s manual that any AR-15 is 5.56mm or .223/5.56mm. I have tested ARs lately that claim, in writing, .223/5.56mm on their website spec sheets and in the owner’s manual that came with the AR. When I called the manufacturer and asked if the chamber is 5.56mm or is it .223, the manufacturer tech help person dismissively told me it is both.

I stuck to my guns (forgive the pun) and said, “It can’t be both; either the chamber is SAAMI spec .223 Rem. or the chamber is Mil-Spec 5.56mm; which is it?”

Then, from one USA manufacturer, I received the response, “Well, it’s a .223 chamber, but you can shoot both through it.” Another USA manufacturer told me, “I can’t put you through to a tech person, but you can email me, and I will forward your email.”

I did email that customer-service person who could not answer my question, and I confirmed receipt of my email and that the rep forwarded it to the “appropriate person.” That was more than a week ago as of the time I am writing this, and still I have had no response. I will not own or recommend any of their ARs for the foreseeable future.

Until next time, I will share with you what Ron Mida, one of my shooting mentors always told me, “Shoot Straight!”

SLRule

Jerry Kraus is a U.S. Army Airborne Infantry veteran and competitive shooter. He has hunted big game in Alaska and Africa. Jerry is a frequent freelance writer published in Soldier of Fortune magazine. Feel free to connect with him on Facebook or LinkedIn

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Comments (251)

  • Jonathon Tuttle

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    I don’t know if I agree that ‘most’ AR-15s are chambered for .223 Remington (the proper designation). It seems most I look at are stamped .5.56. My own DPMS Oracle is stamped .223/5.56. Now, whether that marking is accurate or not is another matter.
    Also, a huge point of contention is the ammunition available. How many times have I seen boxes marked ,.223/5.56’? And how many times have I come across boxes of commercial ammunition labeled .223 Remington and filled with brass that has a Lake City headstamp and no caliber designation (meaning it’s 5.56). Mind you, these don’t come from the likes of Remington, Winchester, or Federal, but the ‘second-tier’ producers.

    Reply

  • Ed Zeiser

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    I was aware there was a slight difference between the two rounds as far as the shape and size of the round. When I bought my AR I made sure that it could use both types of ammunition. I wish that I had read the article first. Thank you for a well written article that brings clarity to the issue.

    Reply

  • Mike

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    I have a Smith and Wesson M&P AR-15 I just went and took a look to verify. The barrel says “Nato 5.56-19″ I have only used .223 through it, and have not noticed a lack of accuracy. can you tell me… is the 19 the length of the bullet in mm. also since the barrel does not say .223/5.56, that Smith and Wesson is not trying to pull one over on me. I can steal that can of ammo without worrying about shooting issues. Thanks.

    Reply

    • Matthew

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      That 19″ is almost assuredly 1/9″ which is the twist rate of the barrel – 1 turn every 9″ of barrel.

      Reply

  • blackdog

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    Great info here. Looking forward to the .308/7.62×51 discussion as I’m a M1A guy.

    Reply

  • Northeast Pete

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    Awesome…..thank you for the research and article.

    Reply

  • CW McKittrick

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    Your article left out one chamber available for the AR 15 rifle, have you forgotten about the newer chamber being currently offered? .223 Wylde? It’s designed to shoot both. 223 and military 5.56 reliably and more consistently than the other chambers. But I’m sure that you would get the best accuracy out of a match chamber with custom or match quality hand loaded .223 ammunition for that rifle.

    Reply

  • Samuel

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    It is as simple as contacting the manufacturer of your firearm. The possibility of liability will be the guide to their offering you the correct information about calibers. I contacted the manufacturer of my M-15 in writing, via email. The answer was forthcoming within minutes. Just dont guess. Go to the horse’s mouth. It only takes a few minutes to get it right.

    Reply

  • Ben Eby

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    Where does the Wylde chamber fit in to this? I was under the impression that it could shoot both rounds but was slightly tighter and therefore theoretically more accurate than the 5.56 chamber while not having the issues of shooting a 5.56 round through a .223 chamber. Is this true?

    Reply

    • Paul Mayhan

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      The Wylde chamber is like the match chamber the author was talking about. It may be exactly what he was describing actually. It’s a long throat .223 chamber that will prevent 5.56 ammo from getting over pressure (that’s what actually causes popped primers) but not so long as to degrade accuracy. It’s a good compromise as it allows the safe use of 5.56 ammo without significant loss of accuracy when using .223. The vast majority of AR-15s being made now use the Wylde chamber for exactly that reason.

      Reply

    • Rick

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      The Wylde chamber is an interesting compromise that has the longer throat of the 5.56 but is tighter around the walls of the case. It should be fine for 5.56.

      If you would like to check a chamber Michi-guns sells both a guage and a reamer that are easy for most anyone to use. If you only have one or two rifles to ream I would suggest a group buy as the reamers are somewhat expensive. They are very high quality though and will cut numerous chambers.

      Reply

  • Donald Newman

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    Sir, thank you for this extremely informative article. I am a military trained armorer, so I fully grasp your ideas and concepts. I just wish future weapons owners would take a few moments to do their research and speak with reputable gunsmiths who can inform and guide them prior to their making a purchase. I know I did, and did not regret it one bit. Mark Tipton, a gunsmith and friend, broke the whole scenario down in a way that I understood the bullet may be the same but the cartridge, and its make up are not. Please keep writing these informative articles.

    Very respectfully,

    Donald Newman

    Reply

  • Caroline

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    Well written article. Explains the difference between the two chamberings for the ammunition, but does not identify the “jump” for what it really is.

    As explained, the slight difference between the distance that the projectile has between the brass, and the riflings is called the “jump” and that small “leap for mankind” (pun intended) allows for just sufficient expansion of the gases driving the bullet to keep from blowing primers.

    Caution is advised when choosing a rifle. Know enough to “ask” for, and get the most versatile weapon for your purpose. Better to be really safe on this one than have a cracked upper and/or blown primer at a really really bad time.

    Reply

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