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)

  • Elton P. Green

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    There is another variation on the .223 or 5.56 chamber. It is called the Wylde chamber. The throat is elongated and slightly over-bored to allow use of bullets weighing up to 90 grains, being set at maximum cartridge length without touching the lands. I think the free-bore is slightly enlarged also, just in front of the case mouth, tapering to a tight fit just in front of the ogive of the bullet, which would give pressure relief while keeping the bullet properly aligned with the bore. This modification allows the chamber to be SAAMI spec in tightness while allowing the use of 5.56 ammunition safely.
    I can, however, tell you that there is nothing wrong with the military M16A2 in 5.56 where accuracy is concerned. I am retired Infantry and used to instruct basic and advanced marksmanship. With the right rifle and a good ‘lot’ of ammunition, the A2 and the A3 are capable of sub minute of angle accuracy with 62 grain ball ammunition. I have shot 25 meter targets for zero while conducting training with open sights where the three shot groups all hit the dot in the center of the zero target’s circle and made one ragged hole using open sights, and have used (unauthorized at the time) scoped A2’s to shoot 100 meter groups that were inside an inch. Unless you’re shooting Bench Rest competitions, the accuracy differences due to the (very slight) differences in chamber dimensions between the 223 and 5.56 will be masked by other environmental factors, provided good ammunition is used. Mil-spec ammunition is not match-grade ammunition, so don’t go by how accurate it is in your rifle. Keep in mind that each lot only has to meet military requirements for accuracy, which is nowhere near match accurate. Your mil-spec chambered rifle may be extremely accurate, but your mil-spec ammunition may only be capable of 2 1/2 inches at 100 meters from a test barrel. And foreign ammo for sale as surplus may not have passed even that requirement. For example, I have fired Turkish ammunition in 7,62X51 (.308) that wouldn’t print inside 5″ at 100 yards in one of my rifles.
    If you want a match accurate AR in 5.56, the chambering won’t affect the accuracy of the rifle significantly. The ammunition used will. So will the type of barrel, rifle manufacturer and the trigger mechanism. First, buy the rifle from someone like Fulton Armory, who makes very high quality rifles in AR10, AR15, M1A and M1Garand configurations, with accuracy guarantees. Then either buy premium ammunition or reload for match accuracy. Also, while the short barrel M4 configuration is a specialty rifle, and of little use past 250 meters, so stick with the 20 to 24 inch barrels.

    Reply

    • Mark

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      Your comments were spot on.
      However – the original article is as misleading as they always are.
      There is no difference between a .223 & 5.56 cartridge. Honestly – there are over a half a billion reloaded cases sold annually made from brass picked up everywhere from military bases to local shooting ranges – who/what would sort these out for the reloading process safely? The .223 was designed to shoot bullets from the 30 – 50 grain range thus shorter OAL needing a shorter freebore. The 5.56 is the same cartridge but loaded with 55 to 77 grain. 99% of military loads are 55G or 62G (M193 or M855) which were designed with a longer freebore = the 5.56 chamber. The pressures are the same for the same load/grain bullet – SAAMI measures mid case and the military (NATO) measures at the neck. Just as with a hosepipe – there is more pressure at the pinch point so the NATO round measures higher and appears “hotter” to people who are drawn to the bigger is better philosophy.. There have been countless double blind tests with both rounds with chronometers and no one has ever been able to consistently identify which round they were firing. THIS is the differece – SAAMI does not test the same way NATO does so they will not comment on using the 5.56. The danger of the 5.56 is that it will most always have a heavier grain bullet which is longer and when you chamber it in the shorter .223 chamber there is less freebore range (length from tip of bullet to the rifling) and thus could build up to unsafe pressures. This would only happen with 77G+ bullets. If you are shooting M193 or M855 you can shoot all day long with no worries – i have done it with tens of thousands of rounds along with my shooting buddies with chambers labeled with dual compatibility and nobody has ever had any difficulties – blown primers – etc… this is the same rifle range cowboy stuff people love to “educate” the newby’s on. Go with the 223 wylde – shoots all available grain bullets safely and more accurately than the 5.56 at longer ranges.
      BTW – a commercial 223 & 556 cartridge hold the exact same volume water and weigh the same and have the same OAL empty – where would the difference come from?

      Reply

    • UncleDano

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      Every thing that I have read about chamber difference is the “throat lead” is longer and a siight different angel. But nothing about the chamber being actually larger. If you look up “cartridge” dimensions for 5.56mm and .223 Rem., you will find they are the same numbers. All the chamber dimensions I have found are for the .223 Rem.
      Where can we get the actual chamber dimensions for the Mil-spec 5.56mm?

      Reply

  • George Dean

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    Counting your dedicated match chambers, there are then four chambers in this family. 5.56; .223; .223 match & .223 Wylde.

    My rifles are 5.56 from Daniel Defense and two M4 uppers with 1:8 Wylde barrels–an 18″ & 20″.

    The Wyldes will safely accomodate NATO 5.56 & civilian .223. Above 77 gr you may need to feed single rounds as the OAl may exceed your magazines limitations.

    I’m sure that you are aware of all this, I post it for those that do not.

    Reply

  • H

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    Great info for the owner/operator of an AR type platform who is unaware of the ammo differences.
    However, at the risk of raising the ire of the Lawsuit Gods, would it be possible for you to actually Name NAMES of the offending platforms? Those who balk at specifying or just down right REFUSE to say because, in fact, THEY DON’T KNOW what they are selling for whatever reason?
    Public “shaming,” if you will, has been shown to be VERY effective at creating behavioral changes in today’s business world and I believe it would be used to good advantage in this instance as well, my Good Sir.
    You’ll be accomplishing several (GOOD) goals.
    One, telling the manufacturer they’ve been “outed” and they NEED to come clean and,
    Two, letting your loyal readers know which manufacturers we might need to avoid like the plague because, for all intents and purposes, they are, very nearly, putting OUR safety in jeopardy by their lack of knowledge or downright bullheadedness and/or ignorance. Even if it is only their Tech Support folks.
    See what I did there? 😉
    Anyway, I would personally appreciate knowing who those Bird Turds are who are so reticent and cavalier about failing to KNOW their business and putting some possibly unwitting customers or innocent bystanders at risk of injury.
    Thank you for your efforts.

    Reply

  • 70's Ops

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    Thanks Jerry. I’ve been trying to explain this for years, but as you stated, the manufacturers perpetuate the myth. Now I can just refer them to your article. By the way, 5.56 is all I have outside the Grendel. I have not noticed any significant difference in accuracy up to 200 meters. After that the hotter load of the 5.56 begins to show itself. With much greater penetration. And slightly tighter groups. Again thanks.

    Reply

  • Alan Carnell

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    Not trusting manufacturers is good advice.

    I have it documented that Remington lied on the specs of a .303 cartridge.

    Reply

  • Secundius

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    That’s why I prefer “Bolts”! I have a “Frankenstein” 98k Mauser, chambered in 8mm-06 with a Japanese Type 38 Barrel “Rebored to accept a .35 Whelen (9.1×63.34mm)…

    Reply

  • SigStu

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    Thank you for a thorough explanation, it’s so hard to find articles where they actually discuss the topic, so thanks, good info.!

    Reply

    • Don Harmon

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      What about the Wylde Chamber. This chamber design is said to be constructed to allow safe firing of either 5.56 mm NATO or .223 Rem. My Springfield AR-15 has such a chamber.

      Reply

    • Mike A

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      I have the ARmalite 15 and the barrel says 223 Wylde and that is exactly as you described it – shooting either bullets without problems

      Reply

  • Types Ammo M16 – Hunt Sodak

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    […] AR-15.223 vs Mil-Spec 5.56mm Chambers – There are three types of chambers in the M16/M4/AR-15/MSR family of rifles, Shooting Mil-Spec ammo through a.223 chamber also may crack your upper receiver;. […]

    Reply

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