223 Rem vs 5.56: An Exhaustive Review

Gun Digest 2013 Cover

Editor’s Note: This article comparing the .223 vs 5.56 is an excerpt from Gun Digest 2013, the world’s foremost firearms annual book. Used with permission. Click here to order Gun Digest 2013 from Cheaper Than Dirt!

by Patrick Sweeney

To a whole lot of shooters, ammo is ammo—if it fits, it shoots. These shooters tend to be the guys with seriously tired, worn, or even busted firearms. They also tend to focus on the wrong thing; you know, the guy who scrubs the brass marks off his ejector lump, at least until one day his rifle stops working or breaks into many pieces.

Ammo is not ammo. And when doing a .223 vs 5.56 comparison, while the loads are almost identical, they are not the same. To know why, we have to go back to the beginning.

The early 1960s were an interesting time. The returning GIs from WWII and Korea had a decade to get things the way they liked. Two tastes they acquired during that time were varmint shooting and benchrest. Varmint shooting was simple. Various members of the rodentia clan, going about their usual business in a field or pasture, served as animate targets. They were prolific breeders, there was no limit, no season, no quitting. You could shoot all day if you wished. Well, as much as shooters then and now like to shoot, shooting varmints with a .30-06 was just silly. The recoil would beat you up, the noise was alarming, barrels got really hot really fast, and the cost of ammo, even back then, was just off the charts.

So they went down in caliber until they found that various rifle cartridges using .224-inch bullets did the job nicely.

Benchrest shooting was a refinement and variant of target shooting. Instead of trying to coax all the shots into a 10-ring, the group was the score. The smaller the group, the better the score. Again, smaller was better, and the common .224-inch diameter bullet served well.

The premier cartridge in the early 1950s, when varminting and benchrest got started and began revving up, was the .222 Remington. Introduced, in 1950, in the Remington 722, it was superbly accurate, and the rifle was also a brilliant out-of-the-box shooter. The mild recoil would not cause a benchrest shooter to have aiming problems, and the mild report, efficient powder charges and low bore erosion made it a useful varmint cartridge.

For those who needed more reach in the varmint fields, Remington came out with the .222 Magnum in 1958, offering 2300 fps more velocity than the little .222.

Now we shift gears from varminting to the on-going soap opera of the U.S. Army rifle situation. Having spent a decade and millions of taxpayers dollars, the U.S. Army Ordnance bureau has brought forth … an improved M1 Garand. And so screwed up is the process that they can’t even produce rifles quickly enough to arm the U.S. Army in any reasonable time frame. I once looked into the numbers and came to the conclusion that, at the rate the Army was buying and building (the U.S. arsenal at Springfield was still open then), the entire U.S. Army would not have been switched over to the M14 before the bicentennial. For those who don’t remember that occasion, the year was 1976.

So, the Army finds, in the mid-1960s, that the Armalite rifle is one that could actually be forced upon them. They pull out all the stops and do everything they can to prevent this. “Real men shoot .30 rifles” was the prevailing ethos of the day (and in some circles, still is).

The cartridge the Armalite rifle was chambered for was the “.222 Special,” a case halfway between the .22 Rem. and the .222 Rem. Mag. It also split the difference between them in velocity. The Army, recognizing an opportunity, first accepted the velocity as sufficient. Then they upped the stakes and insisted on better and better down-range performance. Basically, they kept asking until they had exceeded the pressure limits of the .222 Special. But the problem is that pressure is not simply velocity-dependant. Still, the designers had managed to meet the velocity specs, and the rifle was adopted.

I have now, in less than 700 words, summarized years of work, 100,000 man-hours of engineering, manufacturing and range testing, and we’ve only begun.

Before we get too deep into this, you also have to be aware of a change that happened in our lifetimes (well, the lifetimes of the old farts among us), and that is the change in pressure measuring. If you have an older reloading manual, you’ll see the measuring units denoted in C.U.P., and in some older manuals “CUP” and “PSI” are used interchangeably.

The old way of measuring pressure was known as the copper crusher method. In it, a test barrel would have a hole drilled through it to a specified set of dimensions. Then, a little copper cylinder was clamped in place over the hole. When the round was fired, the copper cylinder got hit with the pressure and was compressed. By measuring the length of the cylinder before and after, ballisticians could determine the peak pressure. This was known as “copper units of pressure,” or CUP, but was often expressed in pounds per square inch, or PSI. The copper (and lead cylinders, used for lower-pressure calibers) could only tell us what the peak pressure was, however, not how fast its onset was, how long it lasted, etc.


Today, transducers, or strain gauges, are used to measure pressure. Here, the gauge, which is essentially a transistor (it is more complicated than that, but we’re discussing firearms, not electrical engineering) is fastened to the barrel. When the gauge is stressed, the electrical resistance of the gauge changes. The beauty—and the problem—with this method is that it is dependent on a computer or other recording device. Depending on how much you spend, you can record the pressure of the event hundreds, thousands, or more times per second. This caused problems in published loading data.

Let’s construct our own cartridge, just so we can remain theoretical for the moment. The “.30 Zoomer Magnum” has a maximum average pressure (MAP, or the allowed peak) of 50,000 CUP. We use the newfangled transducer to measure the standard reference load (in this case, 42 grains of “XYZ” powder under a 183-grain soft-point) and come up with 57,000 PSI. The “new” MAP for the .30ZM is now 57,000 PSI, where before it had been 50,000 CUP. But the actual pressure has not changed, we are simply using a new yardstick to measure it with.

Then we run into problems. In checking loading data, we find that some of the data wasn’t as “clean” as we thought. An example: using “123” powder under the same 183-grain soft-point, we had found that we could get 100 fps more and still only see 50,000 CUP pressure. With the new transducer and seeing things in thousandth of a second slices, we see that, yes, the main pressure peak is only 57,000 PSI, the allowed max by the new yardstick, but we also see a second, higher, spike from the bullet hitting and stalling in the rifling. That spike comes in at 63,500 PSI, well over the maximum allowed. So, we have to throttle back the load data, and all of a sudden “123” powder loses its 100 fps advantage.

The problem came from the copper cylinder not being sensitive enough to register the second, over-max pressure spike, so, no, we have not “slowed down the load data to satisfy the lawyers.” We didn’t know we were going over-max before. We do now, and we have to adjust the data. (Oh, and just to add to the confusion, where you place the transducer can also have an effect on the pressure you measure.)

The SAAMI-spec pressure ceiling, the MAP allowed for the .223, is 55,000 PSI. No, there is no handy-dandy formula that lets you convert old copper-crusher pressures to PSI. The ballisticians tried, and they tried really hard, to come up with a conversion factor. The trouble they ran into was that every cartridge seemed to have its own factor. It was bad enough converting from CUP to PSI, but trying to tell people (and this is just an estimate, don’t use these as numbers to go by) that where they could use a plus-12 percent CUP-to-PSI factor for the .293, the .34-06 used a plus-15 percent, and the .305 used a plus-nine percent. (And, yes, I deliberately used nonsense calibers. Don’t try to decipher them, there is no pattern, nor any useful info beyond what I just told you.)

There was no way to formulate an equation for a “universal translator” of CUP to PSI. Give it up, forget the conspiracy theories your gun club buddy tells you, just accept the new info for what it is.

The NATO spec for 5.56 has a higher “ceiling,” but it’s also measured slightly differently, and, again, there is no handy-dandy conversion. The SAAMI method measures pressure at the middle of the case. NATO (the European measuring group is known as C.I.P.) measures at the case mouth. A CIP-spec 5.56X45, measured at the case mouth, shows a pressure of 62,000. Measured at the case middle, as SAAMI does, it shows 60,000 units of pressure.

But the problem isn’t just pressure. That CIP pressure of 62,000 PSI? It is measured in a 5.56 chamber. If we take the same round, which shows 60,000 PSI/SAAMI (still 5,000 PSI over the .223 max) and put it into a .223 chamber, things get ugly. Really ugly, and really quickly. The pressure spike piles onto an already over-pressure round. I’ve talked to professional ballisticians, guys who use million-dollar labs to measure ammo for their ammo manufacturing bosses. (You know, those guys with the computers and transducers than can measure pressure by the thousandth of a second or finer.) They have reported some instances of 5.56 ammo in .223-chambered pressure barrels demonstrating peak pressures at or above 75,000 PSI. That is the pressure of the proof load each rifle gets tested with at the rifle maker’s, before shipping.

Proof loads, for those who aren’t remembering, are the deliberate, plus-30 percent loads that each rifle maker fires, once per gun, in their rifles before they ship them. They do so in the full expectation that the rifle will do just fine. Once. More is abusive, stupid and asking for trouble.

At this point, many an advocate of “there is no difference” will say “I’ve shot thousands of rounds through my AR and it hasn’t given me any problems.” I’ve worked in gun shops for too many years to accept round-counts mentioned across the counter at face value. Nothing personal guys, but the true number of rounds fired is typically a quarter to a tenth of the asserted number. I teach law enforcement patrol rifle classes in the summer, and I see how much work (and have done it myself) it takes to run 1,000 rounds through a rifle. If your buddy says “Yea, we went to the range this weekend and put a thousand rounds through each rifle,” he’s exaggerating. And if he isn’t, you do not want to borrow any of his rifles, as a thousand rounds in two days is enough to smoke the barrel.

Also, most shooters haven‘t fired enough real 5.56 ammunition to actually test their rifle. Almost all the “generic” ammo you shoot is not 5.56. Oh, it says “.223 Remington/5.56” on it, but it isn’t really 5.56. The high-volume, low-cost bulk ammunition that most of us use is not loaded right to the red line. I’ve chrono’d enough of it to know that much of it falls 100 to 200 fps short of full-book 5.56 spec. That right there is enough to make it no big deal chamber pressure-wise, because the peak pressure of the .223 load is sufficiently less than that of the 5.56 that the artificially-induced spike still falls below the pressure ceiling.

The extra pressure produces faster wear on your rifle. Since most shooters don’t shoot enough to wear out their rifles in any reasonable time frame, the extra wear is hardly noticed. But you can have a serious problem if the variables stack up against you in a range session. Rifles get hot when you shoot them. They also get hot in the summer, in the heat and the sun.

So there you are on a hot summer day, shooting your supply of real-deal 5.56-spec ammo through your .223-chambered rifle. The summer sun beats down and pressures rise. Black rifles left in the sun can easily reach 140 degrees even before they’re fired. Add to that the temperature increases from shooting, and you have some real heat problems coming on. Let’s make it worse: the particular lot of your 5.56 ammo is at the top of the allowed pressure and at the bottom of the allowed brass hardness. The ammo maker tested it in a 5.56-chambered test barrel and, while it was in the top end of the allowed specs, it is within the safety margin.

You’re having a blast, when all of a sudden your rifle stops working. What happened? Well, the heat increased the already maximum-made-excessive pressure and, on extracting a fired case, the pressure had expanded the case enough for a primer to fall out of the primer pocket and into your rifle. Actually, it probably has been losing primers for the last couple of magazines—pick up and inspect all your brass. You’ll see you’ve been losing ne or two primers per magazine. But it wasn’t until one fell into your action and tied things up that you noticed.

How bad can this get? In a patrol rifle class last year, a police officer was pushing his safety back to Safe (and the selector was resisting), when the rifle suddenly spat out a three-shot burst, then stopped working entirely. He’d blown a primer, and the anvil of the primer had wedged under the trigger in just such a way as to create the burst. Typically, the primer wedges under the trigger in such a way as to keep the rifle from shooting at all. Either way, not good.

One solution would be to only use .223-spec ammo. That can be okay, but, if you find a deal on 5.56 ammo, it kind of makes no sense to buy a “deal” you can’t use. Also, some of the best ammo for some applications is 5.56-only. Plus, you can’t control the outside temperature and probably not how much ammo you may need to fire. It would be nice to have a rifle that handled 5.56 with aplomb. But how? To begin with, you have to be able to measure what is there.

The first thing you have to know is this isn’t about headspace. A headspace gauge only tells you the dimensions of the shoulder and case body, not the neck and leade. You need a leade/throat gauge, and for that you need to get a .223/5.56? Gage (yes, the “?” and misspelled “Gage” are the part of the correctly named product), from Michiguns ( I have to be up front and tell you that I have known Ned, the inventor, for nearly 30 years. I don’t get anything but thanks from him for recommending his great gizmo, and I think it is useful enough that I’d recommend it if I didn’t know or even like him.

The Gage is simple and ground to just under the maximum specs of a 5.56 leade/throat. Drop it in and, if it drops free, you have a 5.56 leade. If it sticks (it is hardened steel, don’t pound it in), you have a .223 leade. If you’re curious and want to know just where exactly it is catching, you can mark it up with a felt-tip pen and, with a little careful turning (clockwise), you can see where it rubs. If you are really curious, order some Cerrosafe. Cerrosafe is a special metal alloy with a low melting point. You push a cleaning patch until it is in front of your chamber, heat the Cerrosafe, pour it in the chamber and let it cool. Once cool, you push it out of the chamber, and now you have a cast of the chamber, throat, and leade. You can inspect and measure to your heart’s content.

So, with the Gage or Cerrosafe you find that you have a .223 chamber and you wanted a 5.56. If the rifle is still brand-new, you can send it back. However, the maker probably only has more barrels of the same kind from the same maker, and you may not get a 5.56 no matter how many times you ask. So, you need a specialized reamer. One that cuts the leade and the leade only. (You don’t want your headspace changed.) Ned makes that, also. Now, I can hear some of you saying, “But, I have a chromed barrel, I don’t want to cut the chrome!” Okay, stick with a chromed .223, that’s fine.

But, if you want a 5.56 leade, yes, the reamer will remove chrome. But guess what? The area being cut is the area where the chrome is blasted off first, so if you’ve put more than a few hundred rounds down your barrel, there’s probably not much chrome left there anyway, especially if you did rapid-fire shooting or heated the barrel up to the point where you had to wait for it to cool.

In all fairness, you don’t have to have Ned’s reamer. Other various reamer makers will be happy to supply you with a 5.56-spec finish reamer. You just have to be aware that a finish reamer will also ream the shoulder, if you aren’t careful. So, you may go in attempting solely to make a 5.56 throat and end up creating excessive headspace along the way. Ned’s reamer does not cut on the chamber shoulder at all, therefore, when you feel it stop cutting, you are safely done. It also makes a leade longer even than that of 5.56, though by a small margin.

What’s that, another protest? “But my barrel is marked 5.56, I can’t have a problem.” Alas, that is not the case.

At my latest LEO patrol rifle class, I chamber-gauged the two dozen rifles the officers had brought. All but two were marked “5.56.” One of those was an M16A1 and the other had a completely unmarked barrel. Of the 24 rifles, six failed the .223/5.56? Gage test. Two of those were not just .223-chambered, but clearly on the small side of the dimensions, as I had to use force to remove the Gage.

How can this be? Remember how barrels are made. The manufacturer uses a chambering reamer to turn the chamber out of the back and of the barrel blank. As reamers dull, they are re-sharpened. Each sharpening makes them fractionally smaller. Reamers start at the maximum size and, as they “shrink” from repeated sharpening, the chamber they cut also changes. Once they get to the minimum, they are discarded and a new reamer is employed. Well, some use reamers for a bit too long, and the chamber cut can be at minimum or smaller dimension.

Of those six that failed the Gage, three ended up showing pressure signs later in the class, so we reamed them with the Michiguns reamer and those problems went away. Two of them were the markedly undersized barrels. The other barrels/rifles continued to work, but for how long? They may have been getting fed .223-pressure ammunition, and thus would not show pressure signs.

Having a .223 chamber in your AR is a greater concern than just the social ostracism of having a rifle that is “not Mil-Spec.” However, it is something you can test and fix, if needed. Me, I’ve long-since checked all my rifles, and those that didn’t pass the test have been corrected.

Click here to see our selection of 223 Remington and 5.56mm NATO ammunition.

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Comments (31)

  1. One thing to remember is that 5.56×45 ammunition has crimped primers; (many have primer sealer to boot). So it takes a pretty heavy pressure overload to knock out a primer. Reloaders have to ream the primer pocket out before a new primer can be seated in the primer pocket. Reloaded 5.56×45 cases will not have a crimped primer.

    1. Great Article. Should have mentioned the SPIW program????
      A comment is the French about 1980 got rid of their arsenal system totally and replaced with a Privately Owned Defense Conglomerate with public funding and 50% ownership. This has been very successful at solving the problems of forever development and get noting out. French decided if it took 60 years to field a new design why bother… we are right now at that number the French finally got feed up on with the new infantry weapon they still try to replace the M16 with… actually offshoot of the SPIW… renamed every few years with new requirement docs issued. Picatinny finally issued a couple hundred in a PR campaign to keep it alive, in a dumbed down 25mm grenade version only. Issued Ranger troops patrolling in Afghanistan a couple years ago. Assessment: “Too heavy, reduces squad firepower, exposes soldiers too long while the computer and ranging device works, EXTREMELY HAZARDOUS TO USE !” Got thrown in some dumpster in country after setting around in a pile in the dirt for several months after setting in the armory months waiting Picatinny to arrange transport. Kept getting moved after the Commander tripped over the mess for the umpteenth time, time and again. Cost per unit field tested just about $250K.

      Here is a link to a nice dwg. of the difference
      Nice chart from a Russian Blog.

  2. the new ranch rifles from ruger will shoot 223 or 5.56 ammo. dont let them tell you that the wont. call ruger give them serial number off your gun. the can tell you whats what about your gun. i will tell you that the older rugers stamped 223 will not shoot 5.56 ammo. how do i know this i tryed it you dont wont to know what happen. but i dont have it now but i still have the stock. but like i seed when your not shure about it call the the people that made the gun. dont be a fool and take some stupid advise frome your buddy.

    1. Years ago, I purchased a new Ruger mini 14, It is an early model with a very low serial # and stainless steel hardware. I n the manual that came with it, it specifcally states that it is chambered for both 223, and 5.56 rounds. It shoots both 223 & 5.56 with no problems. Maybe the later models specs are different.

  3. you will haft to excuse my spelling on my first coment. i was in a hurry. you shoot what you wont to in your ranch rifle 223. but rugger did tell me that the new rifle would shoot 223 or 5.56 but like i seed i stick with 223 thats what is stamped on th recever. good luck bu call ruger if you have a ranch rifle and see what the tell you.

  4. i have a new 223 ranch rifle from rugger stam ped 223 i called rugger about usen 5.56 ammo in this meney 14 ranch rifle the told me to use all of the 5.56 ammo i wanted to in this rifle tha all of the new model ranch rifles were built to use eather one and even thow it only was stamped 223 on the chamber the rifle is chamberd to shoot eather one i wanted to use if you have a older meney 14 ranch rifle it wont work for you at least thats what the told me but i still stay with 223 rounds in my rifle.

  5. Good article and comments. The percentage of rifles which didn’t meet 5.56 headspace troubles me. I have an AR that shoots great and is marked 5.56. But I have noticed that when I shoot 5.56 ammo, the brass that is ejected (whether I fire the round or not) is crimped in a castled pattern. Most of it is so seriously imprinted that I cannot use it for reloading. Now I wonder if it is correctly chambered.

  6. I have found that in either my Bushmaster A2 Patrolman or Colt LE6920 they will fire either the .223 or 5.56 ammunition with no problem. Both are stamped 5.56 NATO on the end of the barrel. Now, in my Ruger Mini-14’s, I have a law enforcement and then a tactical model, I only fire .223 ammunition. They are well suited for any situation, as well. Bottom line – buy cheap – you usually get cheap. Blessings of lierty to you all!

  7. i enjoyed the very informative article….. it had many interesting facts and was presented in such a way that even a novice could keep up. I do have a problem with one part of your article : “In a patrol rifle class last year, a police officer was pushing his safety back to Safe (and the selector was resisting), when the rifle suddenly spat out a three-shot burst, then stopped working entirely. He’d blown a primer, and the anvil of the primer had wedged under the trigger in just such a way as to create the burst.”I find this situation either incomplete or inaccurate, the m16 plateform is specifically designed not to fire unless the trigger is depressed. my premise is this :if the bolt was to the rear the firing pin would be retracted, if the bold was in battery or chambed, then the trigger mst be depressed to fire. perhaps you forgot to mention that the “patrolman had his finger on the trigger when trying to place the weapon on safe, and then causing the weapon to fire. this is a very common accidental firing which happens with people unfamiliar with this weapon. i did enjoy your article …….

  8. This responds primarily to comment number 5, remarking about having kept the M-14 and using 5.56 sabot ammo. I do not intend to deal with the wisdom or practicality, nor the potential accuracy, of the proposed solution.

    However, I was on active duty at the time the decision was made to convert our military from a rifleman’s caliber to the “mouse gun”. I recall some of the arguments in favor of the conversion in the debate at the “puzzle palace” on the Potomac and the “Whiz Kids” of the McNamara DOD. At the time, I wondered if they were old enough and had the ability to “whiz”, and if they did, did all their brains leak out?

    At the time, DOD advanced several reasons for the conversion to the mouse gun. First, the difficulty, expense and time required to produce the M-14. That has already been discussed.

    However, the “Made by Mattel” plastic mouse gun lent itself to faster, easier and less expensive production. That made it attractive to a growing military increasingly involved in Vietnam. At least, it made it attractive to the soldiers turned politician in the Puzzle Palace, doing the bidding of the liberal powers in charge at the time. Some men willing trade integrity for position and power.

    Second, there was the issue of cost. The McNamara DOD was all about doing more with less and saving money whenever possible. It took a significantly smaller supply of brass, powder, and projectile material to make a 5.56X45 round in comparison to the 7.62X51 round. Less material meant lower cost and, viola, SAVINGS.

    Third, anyone who has ever supervised or been a major part of the movement of a major military command knows that the god of military movement is “weight and cube”. Reduce weight and cube and you facilitate unit movement. The 5.56 is smaller and lighter (let alone less powerful) than the 7.62. Therefore it was easier and cheaper to move more rounds of 5.56 than a similar number of rounds of 7.62 Make it smaller and lighter and you can ship more of it. More pieces in the same size container and movement officers are happy!

    There was a fourth reason, but it was seldom officially recognized. Still, it was recognized practically and discussed privately. I am referring to the known facts that conscripts going into the military to feed the war machine in Vietnam were recognized as being weaker and, in some cases, smaller then their predecessors in WWII. Further, a great many of the conscripts lacked any knowledge or familiarization with firearms. It was thought much easier to teach the inexperienced and ignorant to shoot the 5.56 instead of the 7.62. The 7.62 was heavier and had more recoil. The pronounced recoil of the 7.62 made teaching precision marksmanship more difficult than using the mouse gun.

    Since in the early days, the M-16 was practically a 300 meter maximum range rifle, precision marksmanship was less of an issue than when the 7.62 was the caliber of choice. The 7.62 was known to be effective to 600 meters and some proficient riflemen could make do very well at 1,000 meters with the 7.62. Until the advent of the SS109 bullet, the M-16 remained a 300 meter rifle, barely able to penetrate a helmet liner at that range. Of course, I am (in this post) ignoring the round deflection in heavy jungle of the lighter 5.56.

    Further, smaller and weaker people could handle the mouse gun but complained at dealing with the 7.62. Why, even women could shot the mouse gun.

    Do not misunderstand. I have nothing against women and I treasure my wife. However, the idea of women in an actual ground combat situation seems utterly ridiculous to me. I was raised and taught that women were a “kinder and gentler” gender. They were not suited by upper body strength, temperament or inclination for the rough and barbarous pursuits of sustained military combat in the field.

    Still, the DOD in its infinite wisdom, decreed not only that the service must accept 100,000 conscripts who could not pass the rather low and rudimentary basic tests required for entrance into the military service of our great nation. Further, in pursuit of some number driven idea of perfection in society, DOD decreed the services needed numbers of perceived minorities in the name of equal opportunity. Never mind if that meant that the more conventional service members were deployed more often, spent more time in combat, spent less time with their families, it was for the worthy cause of equality. How could anyone argue with that? So, the mouse gun suited those who in former times could never have aspired to military service just fine.

    Is it any wonder that we had fraggings and morale problems in Vietnam? Is it any wonder drug usage was rampant? We lowered our standards for admission and we got a military that met a lower standard. Funny how that worked out. To those of you not familiar with the military, perhaps this will help you understand why the military prefers an all volunteer force over conscripts, if given the choice. Mere numbers of warm bodies do not equal success, when compared to elite units of motivated and intelligent war fighters.

    Still, I note (and, am informed) that a certain percentage of our actual combat units in Afghanistan carry the M-14 and eschew the mouse gun in actual combat in open country. Ever wonder why the basic sniper rifle is the 7.62 or larger and not the 5.56?

  9. I have a AR bull barrel varmint rifle chambered in .223 Wylde. I have noticed that I got some leaking and pierced primers shooting a couple different kinds of surplus military 5.56 ammo in it. The same stuff shot just fine in my Colt AR15 Sporter, which has a barrel labeled “5.56 NATO” and a receiver labeled .223, as well as in a PSA carbine with a barrel labeled “5.56”.

    Now I save the 5.56 ammo for the 5.56 rifles.

    There is also a difference in chamber length between the two specs. By most sources I have checked, the base to shoulder dimension is .004″ longer on the 5.56 chamber spec than on the .223 Rem. That is enough to make for very short brass life if resized with a .223 Rem sizing die, then fired in the longer 5.56 chamber.

  10. I’ve been shooting 5.56 ammo in my .223 ranch rifle for years and I have never had a problem so I can’t say whether it will harm your gun or not. But i have shot it and it shoots quite good in my Ruger. That’s all I can say about it not me I have never posted a comment about this.

  11. Guess it’s like the .244/6mm thing. I’ve got some .244 ammo, and almost got an old 721 that might have been my dad’s, but the old man that had it sold it and then told my about it. I did buy his pre ’64 model 94 for $50.00 though, when he offered it to me.

  12. I would not shoot the 5.56×45 in a .233 its not a real good idea.
    I have a 556 Sig Sauer that is made to fire the 5.56X45 but I have .223 and 5.56X45 rounds and they will work if you have a 5.56×45 barrel.

    Patrick Sweeney will tell you that in the shooters log or In Gun Shooter Mag

  13. I own a Colt Sporter AR-15 ( 60’s era), bought some Surplus 5.56 NATO ammo at a gun show awhile back and this stuff jams everytime I tried it. My lower is marked
    cal. .223 SOooooo NOW I use only ammo marked .223, ….. NO problems ! I too thought .223 /5.56 was interchangable in ALL guns. Long ago I owned a Ruger mini-14 Stainless /Steel w/F.S. and used surplus ammo in it without a single problem but I never checked on my ammo very close back then either.

  14. Great Article. I just purchased a Ruger Ranch Rifle Mini M-14 chambered in .223 Remington. I was interested in its 5.56 potential, so I checked their website. In the Ruger customer service FAQ area they indicate you can fire either.223 or 5.56. Go figure !

  15. First of all to everyone asking “what about my gun?”, didn’t you read the article? That was it’s whole point. Get your chamber measured or live in the dark.
    Now, I already knew there was a difference between the two cartridge lengths and pressures. I had my Kel Tec PLR 16 checked immediately after I bought it. It is 5.56 NATO as stamped. I knew about the difference before I bought my PLR because I research a cartridge and a firearm and there histories before hand.
    I’d like to see some info on loading the .223/5.56 cartridge for optimized performance in the handgun length barrels like the PLR 16 and others.

  16. Dave….This is what is recommended: You can shoot any .223/5.56 ammo in a NATO 5.56 chamber. You should not shoot 5.56 ammo in a .223 chamber. There are two main reasons for this advice. The .223 chamber has a lower pressure rating than the 5.56 (roughly 50,000 CUPs vs 60,000 CUPs) and 2.) the 5.56 NATO chamber has a longer “leade” and will accept a variety of cartridges with a longer COALs (Combined Overall length including the brass and the bullet). When you chamber a 5.56 round that is too long for the .223 chambering, you stand a good chance of jamming the bullet into the lands (rifling) thereby creating a situation where pressures can increase dramatically and possibly cause a rupturing of the bolt which can be very dangerous to the shooter. I suspect that most ARs are chambered for the 5.56 these days if for nothing else, liability reasons. If it is not stamped on the rifle (mine is on the magazine well and reads .223/5.56) you could contact the manufacturer just to be safe. Also, I suspect ammunition manufacturers that label their product .223 are building their ammo to .223 SAAMI specs (not NATO 5.56 specs) so the ammo is safe to shoot in either chamber. Not so for NATO ammo. If you’re gun is chambered for 5.56 it is not a problem. Before handloading came along, I shot .223 ammo in my gun for several years and found most of it to be fairly accurate with one particular brand with a specific bullet to consistently print 5-shot groups under an inch. You need to experiment with different manufacturers and bullet styles to find which bullet our particular gun likes. They are not all the same…different barrel lengths and different twists and different tolerances. Jim

  17. I find it very hard to believe that very many 5.56 barrels are actually .223s.Millions of people have built their own ARs. If this was a real problem it would have shown up before now. I came to this article,read most of it,and did not find what I was looking for.Could you tell us what happens if you put .223 in a Nato barrel and vice versa. What about the Wyldie barrel? I may have spelled that wrong. I have also heard one round was produced with an empty air pocket in it.

  18. Terrific article. I have been dealing with this problem since I began reloading about four years ago. I read everything I can can get my hands on concerning reloading. The first rifle I began building rounds for is a Bushmaster Varminter .223/5.56. It is basically a DCM (competition rifle) with a competition chamber, a matched bolt and a 24″ fluted barrel. It has a flat-top with a rail and scope and is sometimes referred to as a “spacegun” in competition circles. (Best group to date is 6 shots into .29 inches at 100 yds) After reading about the differences between .223 and 5.56 chambering, I e-mailed Bushmaster to find out the actual chamber length as the 5.56 is said to have a longer “leade” to accomodate military ammo whch tends to be longer in overall length. The Bushmaster gunsmiths were very helpful and told me 1.) It is definitely a 5.56 chamber and 2.) I should not, under any circumstances, exceed an overall length of 2.260 inches. Bullet and powder manufacturers have also been very helpful. BTW, I have a .308 Savage Model 12 and Savage was not at all helpful due to what they called “liability issues” The gun shoots extremely well but it would have been nice to get some help in chamber specs. I have since purchased a chamber measuring tool so I know where my loads are in relation to the lands and can make adjustments in length based on actual measurements. While I can’t measure pressure, I do cronograph my loads and am able to check for both velocity and consistency (deviation).

    I belong to a private club and we run an annual deer hunting sight-in for non-members and one of the many things we do is to try to make certain people are using the correct ammunition including .223/5.56 shooters. There are a lot of people manufacturing ammo for the .223/5.56…some is very good, some is good and some of it is awful. Be safe, know your gun and know what you’re feeding it. It’s fun to shoot well.

  19. I have a new Colt AR-15 that has 5.56 stamped on the barrel. Do I need to check the bore to see if it is a true 5.56 or 223? I also have a custom 223 for with a heavy bull barrel and the previous owner named it a 223. I do not see any markings on this barrel. Are most after market custom barrels a 223 cal?

  20. Great article, but leaves some questions for me. I have a mini 14 Ruger “223 cal” on the receiver. I have fired a few 5.56’s with no apparent problem yet. I want to shoot BOTH 556 and 223 ammo in the Ruger mini 14. Would it be safe to do so, if I use the reamer listed in the article? In other words, the article does not address firing 223’s in a 556 chamber. Is there any problem there?

    During Desert Storm activation, we ran into another problem with M-16 5.56 ammo. National Guard units had M-16 A1’s and Active forces had M-16-A2’s. The A-2 556 had a hotter load and would occasionally blow the bolts out of M16-A1’s. So we had to check the National Stock Number of ammo issued to assure we got A-1 ammo. I am not sure how we would know which we are getting on the open market that is military surplus.

  21. imagine how many problems could’ve been avoided by simply going to an M14/7.62-to-5.56 sabot (like .30-’06-to-.233 Accelerator, sabot): easier for projectile to transition into rifling, allows super-cheap, or far wider variety of, bullets; far better ballistics than 5.56 in a 5.56 bore; lower projectile-to-bore friction, less heat build-up; overall, M14 tends toward Combat-Riflery-mentality, rather than Pvts on spray&pray so much of the time; much more reliable system from almost every standpoint; but since the M14 replaced the officers’ M1 carbine, the officer corps said ‘give us some little pea-shooter we can carry, on our little stint as platoon leaders’; we couldve kept the M1 carbine for for those non-warrior types (& the warrior types–we didn’t need to get rid of the M1carbine! didn’t need platoon leaders to carry M14); could’ve eventually bullpupped the M14 (as has been done); M16 shoulve been brought in as a long-term prototype, to see if it really made any sense over M14, M1carbine-variants; M1carbine could’ve shot a .30USC-5.56-sabot, probably WAY-impressive ballistics; has been rebored to take a .458socom-like rd in a single-stack magazine

  22. Woody,

    Great article, much of it shot over my head however. I am curious, as I own a Daniel Defense M4 mid length carbine V7 model. DD recommends using 5.56 but says the weapon can accept .223 REM. So, am I safe to assume that this weapon will function more efficiently with strictly 5.56 ammo? More consistent cycling? If let’s say I used only .223 REM ammo in this rifle for years and thousands of rounds, eventually it might do what?

    Thanks for the great article. Very informative.

  23. Great article! Thanks! I am happy my Kel Tec PLR-16 is marked and confirmed as a 5.56mm, because I have a ton of Surplus and mix 5.56 I use in her! The article explains in much better detail what I have to friends over the years. Now we need an indepth article on the .308/7.62x51mm! To Frank (above): Not5 sure of your question, but I LOVE my Ruger SR in 6.8 SPC! My Mil-Issue rounds went through a 4×4 and then took the oak tree down behind it! Literally blew it apart!

  24. Interesting article. I’m ignorant of the difference, have never had an AR or any such military weapon of this caliber, but I do have a Remington 788 in .223, and love it. Makes me a bit unsure of buying any military type gun chambered for 556, or surplus rnds for my 788, although I have fired some in the past without regard, or problems, and I still have some military issued ammo, and blanks. So, I guess in the mindset of longevity to my 788, and personal safety to myself, I should just buy and shoot only civilian brand .223 ammo huh? Sorry, the article was a bit long, dry, and hard for me to understand, but I do understand now, that there is a difference. I’ve ear-marked this thread so I can refer back to further responses, so maybe I can learn more from the rest of you guys. I rely on factory ammo any more, as all my reloading stuff is packed away. I really like the .223 performance, and really, really like the 788s. I also have one in 6mm, and wish I’d gotten a carbine in .308 when they came out, but the loose, rattling, plastic safety, and the sharp, crude extending mags, with a mag release that would hang on your belt, and eject magazines perfectly while shoulder slung, walking to and from the deer stand????? I could have done without those features. Other than that, the 788 almost put Remington in the red, because they were cheaper, and more accurate than the 700s. Anyway, I’m off topic, I know, I’ll be interested to find out more, as others post here. Thanks Woody.

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