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 (195)

  • dan

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    how can I find where to get the best price for an ar-15? there’s so many these days. you always here from someone saying, I wouldn’t have paid that price, you should have checked out so and so. fixing to buy my first ar-15. in 5.56 of course. but how can I find the best price for one? if there is such a way. thanks for any comments on this.

    Reply

  • Larry D. Baker Sr.

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    I bought a min14 that has 5.56 stamped on it. Is it a 5.56 or a 223.

    Reply

  • J.D. Smith

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    Are we all in agreement that it’s OK to run both .223 & 5.56 through a Mini-14? I’ve heard from many different people that yes it’s allright except for the newer ones 2006 and up and I’d like to know for sure from all reliable sources. Mine’s a 1991 and I’ve owned it for a couple of years, really haven’t shot it much but it’s sure a neat little rifle. And I sure don’t want to do any damage to it.

    Reply

    • Gary Holstein

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      JD, I’ve never owned or fired a Mini 14. They are built on something closer to the M-14 action than an AR action. The M-14 is a strong action and everything Ruger makes is well made. Personally I don’t believe it would be any problem, but give Ruger a call. 603-865-2442 They won’t BS you.

      Reply

  • Jefforey

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    What about the 4th type of chambers the “WYLDE”, I have couple of RRA AR’S with a WYLDE chamber. That is one of the reasons I went with RRA because it can safely shoot both .223 & 5.56 rounds. I even talked to RRA before buying them asking this specific question. Between the 2 RRA’s I have shot a few thousand rounds & only had a few get hung up on ejecting, none that I can remember when ejecting a live round. Even though I’ve never had a problem was I lied to by the people at RRA about the gun being able to safely shoot both types of ammo?

    Reply

  • douga7002

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    I reload 223 and 5.56 brass for my Mini-14. What is your opinion regarding the safety of reloading 5.56 brass to 223 ballistics and using it in a rifle chambered for 223?

    Reply

    • Gary Holstein

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      I started to answer this with a quick statement, rather than go on and on like did in my previous statement. But as I started to answer it in my head I kept saying that needs a little explaining. Then that needs a little explaining, and so on. So I will try to answer the question first, then I’ll elaborate. I will however try to keep it short this time. As long as you are not working with maximum loads the 5.56 brass can be reloaded to .223 “ballistics” and fired safely in either rifle. I must immediately explain though that “ballistics” is what the bullet does after it leaves the barrel. The velocity and trajectory can be duplicated with either cartridge but you have to manipulate the load data to do it. The rest of what I am about to say is comparing commercial .223 brass with military surplus brass. Not all brass marked 5.56 is military surplus. Some of it is plain old .223 brass marked 5.56 to market it to the paramilitary types. If you are not sure treat it as if it is military surplus. If your brass doesn’t have .223 or 5.56 on the head stamp, and you are sure that is the caliber, you can be sure it is military surplus. Most ammo manufactured in the US for the US military is head stamped with the initials of the armory where it is manufactured and the two digit year it was manufactured. For example brass head stamped LC 72 was manufactured in the Lake City arsenal in 1972. That being said, why does it matter to a reloader. The .223 and the 5.56 have the same outside dimensions. Since the brass is thicker on military surplus cases there is less case capacity inside. If you use the same amount of powder in a military case you will have higher pressures. If you were loading the .223 near maximum pressures as indicated in a reputable reloading manual, using that same amount of powder in military brass is very dangerous. Most experts and manuals recommend backing down from there posted loads when using military surplus brass. 10%for moderate loads and 15% or more for maximum loads. Then, if that load shows no signs of excessive pressure when fired, you can add 1 or 2 grains of powder and try again. Continue this process until the fired brass shows signs of excessive pressure or you have reached a load you are happy with. If the brass shows signs of excessive pressure back off on the amount powder to the last load that didn’t. If you are not prepared to do this testing when reloading then don’t reload military brass. I hate to sound like a disclaimer but I have to say anytime you are working at or near maximum pressure loads, you are doing so at your own risk. If you make a mistake, you will be the one paying the price. Keep in mind also that some powders are sensitive to temperature changes. A load near maximum pressure that is safe on a twenty degree day in January, may not be safe on a ninety degree day in August. Let us not forget the primers. Not all small rifle primers are the same. The pressures they create can change slightly from one manufacture to another. That change is exaggerated at higher pressures. If you create a load that is near maximum pressure, then change primers, ie. Remington to CCI, CCI to Winchester, etc., you need to back off an the load and work your way up again. By now I bet I have scared off several potential reloaders. Please come back. Reloading is a very safe and effective and fun way to create ammo. If you get a reloading manual produced by a powder or bullet manufacturer and keep your loads in the midrange of the data provided you will create safe ammo the is faster and more accurate then most factory ammo. Then after you gain experience if you want to play with your loads to make them faster, or tighten up the groups even more, or reduce the loads way down to train a recoil sensitive shooter, you can do that safely too. But there techniques and processes you have to follow to keep it safe.

      Reply

    • douga7002

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      Thanks for the feedback. sounds reasonable.

      Reply

  • Cherokee Scot

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    Now that was a pretty informative piece. Hire that guy.

    Reply

  • Bob Shook

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    Please tell me you’ll actually read articles before printing something this ill informed in the future. The only difference in the two is that 5.56×45 and .223 are manufactured under different standards that set different pressure level limits.

    Reply

  • Gary Holstein

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    I admire this man for his service to our country, but after reading this article I’m sorry but I can’t call him a firearms expert. What gives me the right to say that? Nothing really. I am a shooter like most of the others here. Have been for over 50 years. I have been a reloader for over 40 years. I’ve been a firearms safety instructor for nearly 35 years. I was a gunsmith for 22 years. Now for the article. Mil-spec chambers are slightly larger than civilian chambers. Very slightly. The difference is measured in 10’s of 1000’s(.0001). They also have a longer free bore (the distance from the case mouth to where the rifling starts). It’s not so it can handle more pressure. It’s so during combat the weapon will feed reliably in filthy conditions, with ammo that is dirty, or corroded, or other wise mishandled. Military rifles just have to go bang every time. They don’t have to hit a quarter at 300 yards. Sniper rifles excluded. That dirt, dust, mud, water, or even oil can really mess up things, But the rifle still goes bang. Mil-spec ammo is not loaded to higher pressures than civilian. In fact some manufactures have at least one line of ammo loaded to pressures much higher than military ammo. The only real difference between military ammo and civilian ammo is the brass. Since the dawn of firearms, successful military calibers have been taken up for civilian use. Modern brass cartridges are no different. Whether it was the .45 ACP in 1911, the M-2 aka .30-06 Springfield, 7.62 NATO aka .308 Winchester, or 5.56 NATO aka .223 Rem., the only difference between military and civilian ammo is the gauge or thickness of the brass. Military ammo has thicker brass walls so it is less likely to be damaged if or more likely when it is mishandled. Since the outside dimensions of the cartridge are unchanged, the inside capacity is less. The manufactures know this and they use the proper type and amount of powder to produce safe pressures. Reloaders using loading data developed with civilian brass have to be aware of the reduced capacity in military brass, but this is not an article on reloading so that’s enough said there. How much free bore a chamber has varies by manufacturer and has nothing to do with civilian versus military, except for reasons mentioned earlier. I do agree however that military ammo is not burdened by SAMMI specs for overall cartridge length. If you use ammo that you haven’t verified cycles through your rifle without problems you may have a mess on your hands. Or worse! The one thing that really forced me to comment on this was the idea of rechambering the .223 to the 5.56 specs. First he tells us that the 5.56 has higher pressures than the .223 and can damage the upper which has nothing to do with the chamber size, but then he tells us we can remove more metal from the chamber to make it safe. In fact it can be done safely but the logic doesn’t follow with the rest of his argument. By the way I said it could be done safe. I didn’t say it was smart. Anyone with an $80 reamer and a $30 handle and a $30 headspace gauge can take out the extra metal. However the next time you go to the range you will kick yourself in the @$$ for it. You will have screwed up the barrel. It takes a qualified gunsmith with experience in chambering to cut a new chamber and keep it true to the original bore. I’ve already told what his tools cost. Double that for his experience and you will realize it will be cheaper to just buy a new barrel with the chamber you want. You can swap out the barrel yourself with about $40 worth of tools and you really need those tools to properly clean and maintain your AR anyway.

    Reply

  • William Morey

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    As stated in the article, most of the .223 and 5.56 barrels have chrome lined chambers & bores.
    If you ream the .223 chamber & throat to 5.56 specifications you will remove the chrome.
    I suspect that that would not be a good thing.

    Reply

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