This “warning” has been around, and around, for years, but it’s still not always heeded, or understood. Read why and how it matters HERE.

nato stamp
The circle-cross stamp is a NATO-spec cartridge. Your barrel might be marked “5.56” or a more lengthy disclosure referencing its specs. If it’s “.223 Rem.” do not fire a NATO round through it! Your barrel might also not be marked at all. I’ve increasingly seen that. Get it checked. A NATO round will chamber perfectly in a .223 Rem. All exterior dimensions are patently the same, again, it’s the pressure level.

Glen Zediker

I know this is “Reloaders Corner,” but, every now and again at least, I rip open the end of a cardboard factory cartridge box, or five.

I just got finished building up a “retro” AR15 for a new book. Reasons for that are a few, but probably the main one was that I wanted to recollect the one that “got away,” well, the one that I let go. Errant short-sighted judgment, as is common in youthful people. So I built a replica M16A1, circa mid-60s, well, of course, with only two selector stops. At the heart of that rifle is an original-spec barrel, chrome-lined, NATO chamber.

5.56 stamp
This is a NATO chamber stamp. If it’s “.223 Rem.” that’s NOT the same!

That’s leading to this: I opened up a few boxes of “genuine” NATO 5.56 to check it out with, something I honestly haven’t fired for years and years. Dang. That stuff is potent. Over the past several years, the pressure level has increased. Current standard is a little over 62,000 PSI. (NATO is technically measured differently than commercial, but the figures I give here are accurate for comparison.) Compared to SAAMI specs for .223 Remington (commercial) that’s a solid 7,000 difference. (That SAAMI-spec figure has likewise increased over the years, judging from recent test figures I’ve seen respecting commercial .223 Rem.; most references heretofore were max at 52,000 PSI.)

The main impetus for this article, though, came from a recent experience at a local gun shop. I went in search of a sub-sonic .300 Blackout load, and they had one in .300 Whisper. The counter person told me that it was “exactly the same as .300 Blackout, just like .223 is the same as 5.56…” Whoa. Neither statement is true, although Whisper specs are plenty close enough to Blackout that no differences factor in safety or function. However! I didn’t take the time to lecture, but, dang, .223 Rem. and 5.56 NATO are not nearly the same.

First point: do not fire NATO-spec ammo in a rifle with a chamber marked “.223 Remington.” It will, not may, be over-pressure. Reasons have to do with chamber specifications for 5.56x45mm NATO and those for SAAMI-spec .223 Remington. There is a significant difference in the leade or “freebore” cut comparing SAAMI to NATO. That’s the space in a chamber ahead of the cartridge case neck area that leads into the rifling. NATO is radically more generous, meaning “bigger”: longer, more volume. (About 0.150 inches, based on my measurements of bullet seating depths that touch the lands.) There is relatively much more room for expanding gases to occupy in a NATO chamber. In a SAAMI chamber there’s much less room for expanding gases to occupy. The additional pressure is about the equivalent of another full grain (or more) of propellant in the case. Yikes.

high pressure nato
Here’s what happens putting a factory-fresh NATO round through a .223 Rem. chamber. This case is clearly beat. Sure, it might, should, hold up for that firing, but the case is done and the gun took a needless hammering.

nato beat case

There are other little nit differences to pick between the SAAMI and NATO cartridge, and, therefore, chambering specs, but they don’t really factor in a material sense. There’s bound also to be just as many small differences in cartridge dimensions from one maker to the next. I’ve measured enough to tell you that’s true.

Now. What this has to do with reloading (finally, I know) is based on a question I’ve gotten over the years, a concern to some, or at least, as said, a question. And the answer is that you’re better off going with .223 Remington loading data for any ammo intended for “general” range use. That means blasting away on an afternoon. Just because it’s a NATO chamber does in no way mean you’re supposed to run NATO-spec ammo through it! Back it off and enjoy it more.

If you’re relying on a factory-published data manual to give a place to start, or stop (something from Sierra, Hornady, Lyman, or so on) pay very close attention to the test barrel specifications. Clearly, barrel length has a big influence on attaining the published velocities, and some load combinations are going to be worked up using considerably longer barrels than what the most of us have on our AR15s. But the biggest factor is the chamber used in the test barrel. If it’s a SAAMI-spec (sometimes called a “SAAMI-minimum”) chamber then the data should be on the conservative side. Should be. Do not, however, bank on any idea that you should jump straight to the maximum load listed if you’re loading for use in a NATO. There are, always, too many factors that otherwise create more or less pressure (primers, cases, propellant lot, and more).

As time goes by it probably is less likely to encounter a semi-automatic “.223” that’s not a NATO, but it will be marked as such! Clearly, most ammo is used in the most popular guns. That’s not going to be a bolt-action anymore. Make no mistake, though, AR15s exist plentifully that have SAAMI chambers, and I see a lot of aftermarket barrels that are cut with that minimum-dimension reamer.

So what’s a “Wylde” chamber? This is a chambering spec developed by Bill Wylde, one of the early and leading pioneers in the quest for improved AR15 accuracy. It is popular and available, especially in aftermarket barrels. What it is, is a chamber that’s in-between SAAMI-minimum and NATO, leaning closer to NATO. Rumors are true: it’s safe to fire NATO-spec factory loads through a Wylde. The Wylde was designed upon the introduction of the heavier competition bullets with the idea of providing more freebore to accommodate the necessarily longer cartridge overall lengths necessary with something like an 80gr. Sierra, but keep the amount of jump to a minimum with shorter bullets fed from the magazine.

This article is adapted from Glen’s books, Handloading For Competition and Top-Grade Ammo, available at Midsouth HERE. For more information about other books by Glen, visit

23 thoughts on “RELOADERS CORNER: 5.56 NATO: “GO,” “NO-GO””

  1. I have two AR uppers in 5.56 so I can’t comment about any difference on those but I have an H&R single shot in .223 and that shows no over pressure signs at all !! My only complaint is that the twist rate is 1 in 12 so my groups at 100 yards are just less then 2” ! H&R f’d up on the spin rate using old specs! If I drop to light bullets , I would tighten the group. But as for .556 pressure signs, none existing !

    1. Twist rate is specific to weight, length and caliber of a bullet. If your rifle is 1:12″ twist, 55gr and LIGHTER bullets will be just as accurate as those from faster twist barrels shooting heavier bullets. The original AR-15, M16 and M16A1 were all 1:12″ twist and performed very well. My 700 in .223 is 1:12″ and holds 3/4 MOA at 200m with 50gr Sierras.

    2. I’vegot the same issue with my H&R/NEF. Bought it as 30-30 (really good shooter!) so figured I’d send it back to NEF to fit a .223 barrel, which is a by default a “bull” barrel because the blanks they bore are meant to go up to .45. They also offered a trigger job to bring it down to a crisp 2# or so. I was really looking forward to mounting a decent optic and having one dang accurate low budget rifle. No joy…regardless of any ammo I tried from 40 stepping up to 80 gr – could barely keep it on paper at 100 yards. I put the 30-30 barrel back on, pulled the optic off and tossed the .223 barrel to the back of the closet.

  2. H&R didn’t mess up. That rifle was made as a varmint gun designed for 45-55 grain bullets.

    1. Mr. Bullgator, if you go to Berger Bullets website and use their twist rate calculator, it shows a 55 grain bullet traveling at 3,200 FPS needs a minimum twist of 1 in 7.75 to stabilize that bullet! With the 1 in 12 twist of my H&R rifle, BC is compromised 26% and expect to have very bad results with bad groups and keyholes in the target. By the way, do you want to buy a .223 barrel for an H & R? I will sell it to you for a good price!

      1. My old SAKO L46 .222 uses a 14 inch twist and it stabilizes 55 grain bullets at 3,000 fps. It stabilizes Sierra 63 grain semi-spitzers at less than 2,900 fps.

      2. Tommygun, I have never used that Berger Calculator, but if it gave you those results without any error on your part it is “f’d up” as you would put it. I have a Savage 24v with a .223 barrel with a 1-14″twist. It stabilizes 55 grain bullets to a less than MOA group and shoots 40 grain bullets into a ragged hole. The 1-7″ twist didn’t even come around until people started shooting 77 and 80 grain bullets in matches.

      3. Gary, as I said prior, dropping the bullet weight very low to get the best accuracy on the slower spin rate barrels is what you have to do to get it to group.
        PLEASE! If you have a way to get slow spin .223/.556 barrels to group, tell us! America would love you for it!!

  3. Great article. Now I can go ahead with my lawsuit against Remington. You see, I accidentally fired a Nato round in my “223” Remington and my rifle blew up causing me serious injuries. How was I to know that round was not a “223” round. It chambered into my rifle with no problem. It wasn’t like I dropped a 20 gauge shotshell into a 12 gauge. Rifle manufacturers will probably stop offering the “223” with such an inherent risk.

    1. I’m sorry you got hurt Phil, but it is exactly like you dropped and 20 guage shell into a 12 guage. Basic firearm safety has always been to only use ammo that has a head stamp that matches the markings on the barrel. The fact that the cartridges look similar is not an excuse.

      1. Sarcasm. You provided no data / evidence to substantiate your claim. No pressure testing, strain gauge, etc. The facts are; if you obtained various ammunition and various rifles in both 223 and 5.56mm, and proceeded to fire both types of ammunition in those firearms, you would find zero correlation to your claims. You would validate that a given rifle with a given round has higher pressures than other rifles tested, regardless of the “caliber”. And that delta would be less than 10% of the average pressure measured. The data is readily available from many credible sources.

  4. Again an over-simplistic article to further muddy the waters here.
    I recently had to suffer thru fixing a customer’s stove-piping issues with a Mini 14 Ranch Rifle. Clearly marked on the breech “.223 Rem”. Customer was using cheap commercial .223 ammo.
    Jammed frequently. Pressure too low to fully cycle action.
    Tried my own top of specs .223 reloaded ammo. No issues.
    Tried 2 boxes of better quality commercial ammo. One worked, other didn’t. Then I noticed velocity specs on the boxes. 2950 vs 3250 fps. Called Ruger. They told me very emphatically that “all (except one target model) of their .223 or 5.56 spec’d products will handle .223 or 5.56 ammo safely, and are guaranteed to do so using ammo made to SAAMI specs.”

    2950 fps is “normal” for SAAMI spec .223.
    3250 fps is “normal” for 5.56.
    There is NO SAAMI spec for 5.56.

    I looked at fps specs for various American made ammo. Federal gives the SAME velocity spec for their .223 and 5.56 ammo . 3250 fps. Can this be done at different pressures. Don’t think so!

    Bottom line FROM RUGER’s TEST LAB FOLKS is: “Use full velocity/pressure loads”

    When I did, customer’s gun ran fine.

    Lots of time/energy/money wasted.

    If Federal can make .223 AND 5.56 ammo that works, why can’t the rest?
    What’s REALLY true about this burning issue?
    Can there aver be enough education to overcome this BIG(?) question?
    Why do so many “experts” keep re-stating the same old stuff and get published?
    The internet is a great knowledge base. Some of it is even true. Little of it is complete and true.

    1. I’ve owned a Ruger mini 14 Ranch rifle for 30 years. Have never known it or the other two my brothers had to have an issue with too little pressure. Went as far as machining a smaller bore gas bushing and an extra power recoil spring. These things eject spent cases into the next zip code. I’ve run 5.56 to cheap Russian ammo and reduced .223 hand loads which were very accurate. The factory manual mine came with calls for either .223 commercial loads and 5.56 NATO ball rounds. With a receiver stamped .223 is a little misleading. I have read were some non-ranch rifle models are .223 only, listed by serial number ranges. Do your research prior to blowing something up.

    2. Well said. SAMMI gave a notice out LONG LONG ago about not interchanging but that was a issue of commercial 223 throats being short . That was taken care of 30 + years ago any rifle capable of handling 223 Remington will safely handle 5.56 .

  5. I keep it simple. I don’t shoot commercial ammo. Due diligence during my load work up and all is right in my world. After a proper OCW(optimum charge weight) test, I have a load the cycles my action correctly and prints concentric shot groups centered on the point to aim.

  6. WRONG ! Pressures between SAMMI , CIP and NATO are the SAME This has been a common mistake for years, SAMMI states in their manual 223 has a 55,000 psi and 52000 cup. NATO and CIP have different location and methods that have changed over the years which has show different pressures and times confused the issue by stating CUP was in psi ! If you really believe all of this non-scene take MILITARY Maltese Cross ammo chronograph it and then some good hot commercial ammo and compare the velocities out of the same gun !

  7. Many commercial rifle makers have modified the throats of their .223 guns to accept 5.56 NATO ammunition.

    The ‘older’ 52,000 PSI figure was not PSI. It was CUP. The SAME ammo tested by the two different protocols will produce different figures just as will European CIP protocols.

    1. From SAMMI:

      In Firearms Chambered For Do Not Use These Cartridges
      223 Remington 5.56mm Military
      222 Remington
      25-45 Sharps
      30 Carbine
      300 AAC Blackout

  8. Thought the article was a good explanation of the two.
    IMO there are a lot of folks out there that don’t know nearly as much as they think they do. Newbies for the most part.

    And to the comments above on bullets. Bullet weight is only part of the equation. It’s about length and design as much as anything. So saying a 55 grain as a generic weight is not correct.
    Then to use one manufacturers chart for “all” bullets is another mistake.
    Kind of like burn rate charts. Very few if any powders will be the same number on various powder companies charts. They are there for reference only!
    Never assume.

  9. From SAMMI:

    In Firearms Chambered For Do Not Use These Cartridges
    223 Remington 5.56mm Military
    222 Remington
    25-45 Sharps
    30 Carbine
    300 AAC Blackout

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