5.56 vs .223: What’s The Difference Anyway?

IMI Systems 5.56 ammo (courtesy thetruthaboutguns.com)

What’s the difference between 5.56mm NATO and .223 Remington, anyhow? The rounds are supposed to be interchangeable…kinda/sorta/maybe…right?

The difference in the round itself: the NATO round is a little hotter. The case is the same size, but the powder charge, projectile and also rifle specifications differ. As a result, make sure to check your AR-15 to see which it’s chambered for, as not all AR-15 rifles sold to civilians are the same.

NATO, you see, develops standardized loads so that any member nation has specifications they make a round to. As a result, a NATO round is a NATO round is a NATO round.

The standard loading is NATO SS109 (aka M855) which uses a 62-grain projectile. The powder charge for that round is on the hot side, which is loaded to chamber pressures around 62,400 psi in a rifle chambered for 5.56mm NATO. In a rifle chambered for .223 Remington, the round produces chamber pressures upward of 70,000 psi, if going by SAAMI (the Sporting Arms and Ammunition Manufacturers Institute) standards.

By contrast, .223 Remington tends to produce lower chamber pressures between 52,000 and 55,000 psi, going by SAAMI specs. Therefore, 5.56mm NATO ammo isn’t safe for use in a rifle chambered for .223 Remington, but .223 is OK in 5.56. Thus, powder charge is the main difference between 5.56 vs .223 cartridges.

Wait. How can there be that much more chamber pressure when muzzle velocity for a 62gr pill – according to specs from Federal Ammunition for 5.56mm and .223 – are exactly the same?

The JMT Gen2 composite / polymer lower staying strong as an ALG Single Chamber Brake (SCB) paints the range with CapArms 69-grain .223 Rem SMK.

NATO specifies that any rifle chambered for 5.56mm have a longer leade than that employed for the .223 Remington round. The leade is the distance between the case mouth and the point at which the rifling begins in the barrel.

The .223 Remington chamber dimensions create a tighter seal on the projectile, and thus produces more pressure despite firing the same size of projectile from the same case, given a slightly hotter powder charge, since more gas escapes with the larger leade.

The length and diameter of the leade – called the “freebore” – is longer and wider for the 5.56mm round, meaning there is a wider, longer free space the bullet travels in before it contacts the grooves in the barrel.

In addition to the higher pressure, NATO specs also call for any rifle made for use by NATO forces – including the M16, the M4 and other service rifles like the L85A2 of the UK or France’s FAMAS – have rifling with a 1:7 twist, or in other words 7 inches for a full 360 degrees of rifling rotation. This is so the NATO L110 tracer round (which has a longer projectile) can be fired from the same rifle as the standard ball round, as the optimal twist rate for the 62gr 5.56mm round is more like 1:9.

Most civilian AR-15 pattern rifles have a twist rate of 1:9, which is actually something of a compromise. The slower twist rate enables the shooter to use different bullet weights, whereas a NATO-spec gun pretty much uses 62gr ball and 77gr tracer rounds only.

The civilian shooter could, conceivably, shoot a mix of 40gr varmint rounds, 50- and 55gr ball at the range, and even 62gr ball for when they’re feeling squirrely. Lighter bullets benefit from a slower twist rate (1:10 or even 1:12) so a twist rate of 1:8 or 1:9 makes it so you can get decent performance from anything.

So…5.56 vs .223. The former uses slightly more powder. Rifles chambered for it must adhere to certain specifications to suit it, as it is a standardized military cartridge. The latter also adheres to different specifications (such as a shorter leade) but has a much more diverse array of loadings from available from ammunition manufacturers. Case dimensions, however, are the same.

What this means in practical terms is that you won’t get the most out of actual 5.56 NATO ammo unless your rifle is made to NATO specifications such as the leade, twist rate, and can take somewhat higher chamber pressures. Thus, with its lower pressures, .223 can be safely used in a NATO-spec rifle in 5.56mm, but not the other way around.

Which brings us also to the .223 Wylde round. This isn’t actually a bullet. Instead, it’s a set of specifications that enable a rifle to use both cartridges.

Put more accurately, it’s actually a .223 Wylde chamber. It was devised by one Bill Wylde, who came up with a solution allowing for safe use of both bullets in the same gun while maintaining accuracy. This is accomplished by resizing the chamber for the same length of leade of 5.56mm NATO, but with the freebore diameter of .223 Remington. Either caliber headspaces correctly, preserving accuracy for both.

Now…which one should you get? The NATO cartridge or .223? 5.56 vs. 223?

The answer is you should first consider what you want it for. A gun, any gun, is a tool, ultimately, so therefore consider your task.

The .223 Remington chambering is the better all ’rounder. You can do a bunch of shooting at the range and in competition. You can load a bit heavier for hunting predators or varminting and – since most .223 rifles have that slower twist rate, shorter leade and tighter freebore – the .223 will likely be more accurate.

This means it will be more precise at long range, though there are better rifle chamberings purely for long-range target work and varminting (.22-250 and .243 Winchester come to mind). There is also a better selection of ammunition for hunting other game – if you live in a jurisdiction that allows it – and for self-defense.

5.56mm NATO has fewer loadings commercially available and is also a dollar or two more expensive per box…but you get to say “mine uses military-spec ammo.” In terms of real-world capabilities of the round, that isn’t worth…well, basically anything. Is that worth much to you?

Let us know in the comments what YOU think about that.


  1. avatar RA-15 says:

    Off subject 🙂 6.8 spc anyone ?

    1. avatar What would Spock say says:

      Best intermediate battle rifle round ever devised.

      1. avatar CA ST says:

        That credit belongs to 224 Valkyrie.

        1. avatar Eric Zompetti says:

          And prior to that, 6.5 Grendel.

        2. avatar Umm no says:

          Grendel is a capacity limiting long range cartridge. Valk is a glorified 1000rd prairy dog cartridge. I think we have different definitions of what intermediate means. Both have their place but these are niche solutions, and not remotely intermediate.

  2. avatar Marcus says:

    There’s almost no risk of shooting 5.56 out of any .223 rifle so long as it is in good condition otherwise we would hear about a KABOOM just about everyday from some idiot who has done this which I bet is a lot! All that being said every new rifle should be .223 Wylde and we can be done with this discussion once and for all.

    1. avatar Pete says:

      ^ This. Has there ever been a documented case of a 5.56 round damaging a .223 chambered rifle?
      One would suspect that at most there would be an increase in wear due to the higher pressures.

    2. avatar Ralph Jones says:

      When dealing with firearms, if you know your shit… then you should know that “almost no risk” is never acceptable. Accidents with firearms are never one single mistake, but a compilation of several mistakes culminating in a disastrous result. If you know firearms, then you should know that “almost no risk” is unacceptable under ANY situation. It’s simply contributing to the possibility.

      When dealing with firearms, “almost no risk” is never acceptable. If you’re willing to take that risk, then you have no business handling or operating firearms.

  3. avatar Bllinoteeth says:

    Eh the article answers itself with the wylde chambering. All my rifles are 1/8, nitiride, nib extensions. The common conceptions of barrel specs are changing with new manufacturing processes and newer bullet designs. think the newer 77 grain OTM is great, but expensive compared to 62 gr.

  4. avatar Whoopie says:

    Yeah, yeah and supposedly the .308 is different from the 7.62 NATO. And yet I never heard of anyone’s civilian rifle blowing up because he fired a military cartridge. I also doubt anyone manufactures an AR pattern rifle that isn’t made to military spec. So can we just put this dumb debate to bed?

    1. avatar Joe in NC says:

      Its the opposite. Commercial .308 are usually loaded hotter than 7.62 NATO.

      1. avatar Craig in IA says:

        True. The .308 Win already existed before NATO adopted the basic round.

  5. avatar C.S. says:

    My understanding is that it is even more complicated, because the way the pressures are measured are also different…

  6. avatar possum says:

    5.56 the cases are thicker, they hold a little less, powder but are loaded to slightly higher pressure. I shootem both in Mini14 no problem.

    1. avatar DrewN says:

      My idiot brother in law borrowed my cut down Mini “Target” which is supposedly .223 only and ran about 1K of 5.56 through it. No overpressure signs on the brass, he said it ran fine. It absolutely hates steel case of either flavor though.

      1. avatar possum says:

        I’d have to agree with your Mini, I don’t like steel case either. Besides extractor wear they build up a lacquer in the chamber.

  7. avatar Ing says:

    I went with 5.56 NATO and a 1/8 twist rate because I wanted to be able to shoot whatever ammunition happened to be available with no worries.

    If I’d had more time and/or a larger budget, I might have held out for .223 Wylde for the best of both worlds, but when you’re talking about a $400 AR build, it’s not likely you’d notice a difference. I doubt I’m good enough to notice whatever difference in accuracy the Wylde chambering might impart, anyway.

    It may not be superb at anything, but it’s super reliable and it’s good enough to do everything I’ll ever ask it to do.

  8. avatar kenneth says:

    I would add that one sacrifices much less accuracy by over stabilizing a projectile than under stabilizing. A projectile that isn’t spinning fast enough to be stable will tumble end over end in flight, destroying accuracy at all but very short range. In contrast, I’ve shot 40 grain bullets out of 1/7 twist barrels with almost no loss at all. Not even measurable until past 300 yards.

  9. avatar Kroglikepie says:

    Whoa. Dude. You are WAY off base on a lot of this info. First of all, NATO and SAAMI measure pressure differently. NATO EPVAT is not comparable to SAAMI or CIP methods as the pressure is measured at a different location of the chamber. Second, a 1:7 twist will not be incompatible with lighter bullets. You will have a different POI however. Third, 223 rounds can not be more accurately loaded with heavier bullets, while still having lower twist rates. If the bullet weight goes up, so should the twist rate. Fourth, the only part of the rifle that matters for the chambering… is the chamber. Everything else between 556 and 233 is the same…

    I could go on, but I’m not getting paid to rewrite your article. A lot of this is basic info, and you should probably study up before writing about it.

    1. avatar 16V says:

      I know off-the-top that you’re correct about EVPAT being different than SAAMI, so I’ll assume you’re right about the rest.

      I applaud your knowledge, but correcting the slopfest part-timers around here is a Quixotic task. They (barely) know something about the topic they’re “writing” on, and god forbid actually look up the details (remember, like you did in HS?). Trade Dress anyone?…

      I’m sure this exchange will either hit the memory hole, or be dismissed (as is done nowadays) as “gosh, whaddya think, I’m supposed to know something? Ha!” by the
      cut-n-paste hack “writer”.

      RF’s second creation dies the same death as his first. Oh well…

      1. avatar Kroglikepie says:

        Seriously. This site is turning into amateur hour without the benefit of a dedicated peanut gallery. And what happened to the daily digest?

        1. avatar Big Bill says:

          And “Notify me of follow-up comments by email.” still doesn’t work.
          Is there even a cookie to fill out the comment info any more?

    2. avatar Dyspeptic Gunsmith says:

      It’s even more confusing than that.

      NATO EVPAT testing seeks to measure pressure from the moment the primer is hit until the bullet leaves the bore. They’re not seeking to just measure “maximum average pressures,” or “proof pressures,” etc, as SAAMI and CIP are seeking. The EVPAT test is not only seeking to measure pressures for safety of the operator of the weapons system(s), they’re also seeking to confirm the functionality of the ammo – eg, “this round will penetrate a exemplar helmet at 500 meters…” and so on.

      CIP is seeking to insure shooter safety. The European countries have proof houses, unlike the US. EU civilian weapons are proof tested before they’re allowed to be sold to consumers, and CIP is a joint group of proofing labs/houses that seeks to specify not only what the nominal operating pressures should be, but also what the proof load pressures should be.

      The US doesn’t have a proof house; the closest we have is a private lab that performs testing on firearms as part of their overall service package (HP White). SAAMI isn’t sponsored by, or associated with, the US government or the DOD. It’s a group of arms and ammo companies that co-operate to come up with safe load specs and chamber specs for sporting firearms.

      Because it is a sub-organization of NATO, EVPAT doesn’t have the huge library of information that either CIP or SAAMI has – EVPAT is concerned with only those cartridges of small arms that NATO uses in military forces – eg, the 5.56, 7.62, 9×19, etc.

  10. avatar Timothy V Noecker says:

    I Just Prefer 77gr .223 Myself, Although My RUGER AR-556MPR is chambered to safely shoot both 5.56 and .223.

    1. avatar Micah says:

      How do you like that Ruger MPR? Someone local is selling one for $550 and I’m thinking of going for it.

      1. avatar Timothy V Noecker says:

        Micah, If You Can Get a MPR For $550 Do It, That’s an Absolute Steal Of a Price For an Above Average AR-15!!!
        My Ruger Has S&W M&P15 Performance Center Specs Yet It’s Sold At Half The Price!!!
        The Money I Saved By Buying The Ruger Over The S&W Allowed Me To Add a Leupold VX-R Patrol Scope and Burris P.E.P.R. Scope Mount!!!

  11. avatar Rokurota says:

    My FFL told me a rifle stamped 5.56 (instead of MULTI) should only be fed 5.56 (no .223). That sounded dubious to me.

    1. avatar pwrserge says:

      Your FFL is an idiot. He was probably looking at the receiver stamp. For AR platforms, the chamber specs are on the barrel.

      1. avatar Rokurota says:

        I did look at him like “whaaaaaa?” but this kind of stuff happens all the time, maybe because I look like a rube. Old guy at a gun show tried to get me to buy his overpriced lowers, emphasizing they were MULTI CALIBER.

        1. avatar Widdler says:

          Could have been for liability reasons, either way he should’ve given you the (wink)

        2. avatar Shay says:

          My barrel is stamped 556 NATO 1/7 so with the 1/7 twist and I want to shoot hogs I’ve read that I should shoot 62gr.223 Federal Fusion MSR barrier-blind bullets but I’m still confused because that is 223 not 556. Is one better than other for stopping hogs say maybe 50 yds guessing?

    2. avatar Sittingonadockbythebay says:

      Rokurota, like posted already, your dealer is an idiot or trying to scam you. That means nothing.

      I haven’t purchased a lower in years that did not say multi, Spikes, Anderson, DPMS, etc.

      You bring up a good point. Do not go by what is on the receiver when dealing with AR pattern rifles. Always go by what is on the barrel. Always look.

      One of my older eceivers says .223 REM on it, however it is chambered in .300 Blackout.(Got a factory marked dust cover that clearly says .300BLK for safety.)

      Another, a factory Colt from the 1980s, has .223 on the receiver and the factory barrel is a chrome lined 5.56.

      With the ability to easily change calibers in our ARs, we must be diligent when shooting, especially with new guns and those borrowed from friends, family, stored for years, etc.

      Also a fan of the Wylde chamber.

      Safe shooting everyone.

  12. avatar little horn says:

    this article can be summed up in a few words: chamber pressure/case wall thickness.

  13. avatar Dyspeptic Gunsmith says:

    Hmmm. Lemme pull out some of that high-powered math they taught me in engineering school…

    What’s the difference between 5.56 and .223?


    FWIW, all the AR’s I build for myself with “barrels from blanks” I chamber with a Wylde chamber. It works well, I don’t have any issues thereafter.

    That said, the big issue in .223’s when building a rifle are the twist rates. If I’m recommending a barrel for a p-dawg shooter who wants to shoot 40 to 50+ grain “he blowed up reeeeaaal goood!” varmint bullets, then we’re in the 1-in-12’s and 1-in-10’s. If we’re talking of someone who wants to run 77+ grain bullets in an AR? 1-in-8, possibly 1-in-7. If they’re seeking to single-load the heavier bullets, 1-in-7.

    In the .224 bullet diameter, you now have such a span of bullet weights and lengths you need to think about what you want to do with the rifle before you settle on a twist rate. This applies to the custom .22-250 (and other) varmint rigs just as much as it does AR’s.

  14. avatar BC says:

    The twist rate has to do with length, not weight. Now usually a heavier bullet is longer, but with different materials, a light bullet can be longer. Additionally, you’re not going to notice the twist rate between 1:9 and 1:7 with a long bullet until past 500 yards. So for most purposes, you’re fine with either. Everyone gets all worked up that 1:7 is the best because it’s “milspec” when they would never see any difference.

    As for the risks of kaboom, that really is only a worry for something like a thin walled chamber bolt action. I’d venture to say 99% of rifles on the market today are fine to shoot either, regardless of official chambering.

    All in all, these days it’s much ado about nothing.

    1. avatar Dyspeptic Gunsmith says:

      Well… mostly correct.

      I could type up a detailed explanation, but it’s easier to just give people a pointer to a paper with the mathematics:


  15. avatar pwrserge says:

    People always mention that high twist rates can have a negative impact on your rifle accuracy. I have a mk12 clone with a 1:7 18″ barrel shooting 55gr pills that prints 1″ groups at 100 yards all day with very basic optics and my own marginal skill. Now if you listen to all the “experts” 1:7 is “too much” twist for mil-sup 55gr 5.56×45… I have yet to actually see any significant difference in group size between pushing 77gr SMK and 55gr el-cheapo.

    1. avatar Widdler says:

      Same here, mines a 1:7 and my shooting buddies are all 1:8s-1:9s. 1″ at 100 using 55gr all day long, if we didn’t know you couldn’t tell who shot what

  16. avatar Tim says:

    Wait: “223 Remington” is cited twice. Once as having hihher pressure than 5.56, and later it’s cited as having lower pressure than 5.56.

    Fix, please.

    1. avatar FreakinPeanuts says:

      yeah i had to read that a couple times. got a little confusing. 2 days later and its still not fixed. I am surprised i haven’t seen more people calling it out.

    2. Thank you Tim and FreakinPeanuts. I was just going to ask about that. I read those 2 conflicting paragraph over and over….more pressure with .223…less pressure with .223. Maybe the author isn’t reading the comment section

  17. avatar dh34 says:

    Wouldn’t pretty much all AR15s be “civilian”? They aren’t used by the military (M16 or M4), though some are used by civilian LE agencies. I think as POG it’s important to be consistent, since the antis continually refer to AR15s as military grade weaponry.

    1. avatar goober willy says:

      “military grade” Is Lowest Bidder quality. Semis are used by the military in limited applications. The Civillian “Ar-15s” actually have wide quality range, usually it’s better than expected. Barrels are light years more better then what the military has in stock.

  18. avatar Southern Cross says:

    I use 1:9 barrels with my No4 .223 conversions. I’ve used 50-70g bullets but typically use 55-62g bulk purchases. 69g Sierra Match Kings are nice but at over 60cents each are too expensive. I’ve almost finished the bulk surplus 62g SS109 projectiles and I wish I could buy more. At 3c each they’ve been a bargain.

  19. avatar Leadsled says:

    NATO brass and 223 brass are two different animals.
    If you load 5.56 brass with a 223 Remington load you will have a bad day.
    The case volume of 5.56 is less. The brass is thicker.

  20. avatar Scoutino says:

    “Either caliber headspaces correctly, preserving accuracy for both.”

    What does headspace have to do with it? As you said, dimensions of the case are identical and both cartridges headspace on the shoulder.

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