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!”


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 (262)

  • dan


    where’s the cheapest place to get 5.56/.223 ammo in brass cases?


  • Derek


    Actually the 5.56 has a thicker case: Military cases are made from thicker brass (to keep from splitting the case), which reduces the powder capacity due to higher pressures. This difference in the point of measurement can account for a difference of pressure measurement of 20,000 psi or more. Your gun needs to have a NATO or MIL-SPEC chamber, which features a longer leade. The Wylde and Armalite chambers are the exception. They designed these two chambers to handle both 5.56mm and .223 equally well. Conversely, if you have a 5.56 stamped on the barrel, you can fire both the 5.56 and the .223 equally well.


  • Paul Smith


    There is a tool used by machinists called a “ball gauge” which is used to measure the diameter of holes. The tool is inserted into the hole, or chamber, opened by turning the control screw, removed and the outside diameter is measured with a micrometer. There are also ball micrometers available for purposes like this. Has anyone ever thought of trying this to get a definitive answer on the supposed difference in chamber size between 5.56 and .223?


  • Range Envy: Ten Really Cool AR-15s


    […] AR-15 .223 vs. Mil-Spec 5.56mm Chambers: There are actually three types of chambers in the M16/M4/AR-15/MSR family of rifles, but most people think there are only two. […]


  • Bud


    The 5.56 military SS109 round does fire at higher pressure than the civy .223 round. The opposite is true with the 762×51 military round. The .308 has more juice.


  • Gary H


    I keep hearing talk about the differences in chamber size between .223 and 5.56. If anyone has any official documentation showing these differences I would like to see it. I could be convinced that military ammo is manufactured to higher pressure than civilian ammo(I could be convinced. I’m not yet.), but I don’t believe there is any difference in chamber size. If the 5.56 was even a few thousands larger then the .223 it would not chamber in a .223 and allow the action to close. Also the .223 might chamber in the 5.56 but the headspace would be to much and cause dangerous gas blow by. I have checked three different reference manuals and they gave the cartridge or chamber dimensions for the “.223/5.56nato” round. Right now I want to repeat something I said in an earlier post to this blog. If you have ammo that has 5.56 on it, it is not military ammo. At least not made in the USA. Ammo made in the USA for our troops does not have the caliber stamped on it. It has the armory where it was manufactured and the year. For instance ammo manufactured in 1977 at the Lake City armory is stamped LC 77. It doesn’t come in fancy cardboard boxes either. It comes in bandoliers packaged in ammo cans. Anything else is just paramilitary marketing. This is one of those issues that has been told as truth for so long that everyone assumes it has to be true. Even a host on a shooting TV show mentioned it. When the guest for the show questioned him on it, the host said it was common knowledge. When the guest wanted proof, the host said he would get and edit it in at the end of the show. The show ended with credits which were followed by the disclaimer “It is up to each individual to determine what ammunition is safe for their firearm”, I’m sure if the host could have proven the differences between .223 and 5.56 it would have been in the show. Not the disclaimer, which by the way has not been on any other episode I have seen. One final not on pressures. Higher pressures, while creating higher velocities also create potential problems. Higher pressures cause the brass to expand faster and more firmly against the chamber wall. This requires more force to extract and a stronger extractor. Military ammo is manufactured for semi, or fully automatic weapons that have to function every time without fail. Every failure has the potential to represent a dead soldier. While the AR is becoming more and more popular in recent years, the commercially available .223 has been predominantly a varmint cartridge fired in strong reliable bolt action firearms. You decide which is more likely to be manufactured with higher pressures. Thanks for taking the time to read this post. For more detailed information on this subject look up my previous post early in this blog. Gary


    • Vinny


      The dimensional specifications of 5.56 NATO and .223 commercial brass cases are identical. The cases tend to have similar case capacity when measured, with variations chiefly due to brand, not 5.56 vs .223 designation. The result of this is that there is no such thing as “5.56 brass” or “.223 brass”, the differences in the cartridges lie in pressure ratings and in chamber leade length, not in the shape or thickness of the brass. The 5.56 chamber allows the 5.56 (hotter load) to be fired safely creating about the same pressure found when firing a .223 round in a .223 chamber.

      It is that simple and nothing else needs to be said other than it is not completely safe to use 5.56 ammo in a weapon designed for .223 only.


  • Comanche


    Now add the the confusion the .223 wylde barrel that splits the difference of .223 and 5.56 nato chamber.


  • fishunter


    Thanks for this article. I have been asking the 5.56/.223 question from many people (even sporting goods stores) and have not received a good answer. This article clarifies the what and why! Good job and good shooting.


  • Dan


    If the barrel has .223 stamped on it, then you can NOT fire the 5.56. If it has 5.56 stamped on it, then you CAN use both rounds safely. Having both weapons is fine, especially if the twist rates are different, which allows you to fire 52 grain up to 77 grain. I hope that’s the case with your weapons.


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