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Ryan asks:

Hello, After reading the reviews on the Smith & Wesson MP-15 from your site, I went and got one. I was torn on which make and model to buy. But your reviews and other info I got sealed the deal. I have one question concerning ammo. I was always under the impression, that what ever was stamped on the barrel was the only caliber to be fired through the gun. Mine is clearly stamped Nato 5.56 there is no stamping that it can fire 223. I was told when I bought it, that it can fire both 5.56 & 223. I hear different tales on what is right and or wrong. Please help clear the air for me.

Believe it or not, I get this question a lot. So let me lay it out for you . . .

All ammunition is produced to a given specification, which describes everything from the case dimensions to how far the bullet needs to be seated in the case neck. But the most important of these specifications is the maximum chamber pressure metric.

Barrels, bolts and other components are designed to contain the pressure from the expanding gasses in the cartridge as the gun goes off, but they are only designed to withstand a certain level of force. If the pressure in the chamber exceeds the design limits, the parts can shear or rupture. In other words, the gun explodes. Allow me to illustrate:

Yeah, not pretty. It was a nice Cav Arms lower too.

Anyway, in order to keep everyone on the same page and make sure that the parts can handle the load, manufacturers use a set of common specifications. Unfortunately, thanks to the military there are two different standards for the AR-15’s standard cartridge. And they’re measured differently, too.

In the beginning, there was the .223 Remington cartridge which was a version of the .222 Remington cartridge tweaked to work in the AR-15 design. Then the military took it and jimmied the specifications a little further to make it suit their needs. Since the .223 Remington cartridge was already in civilian use and registered with the civilian specification authority (SAAMI), it was stuck and wasn’t able to be changed to match the military specification when it came out. The two major changes between .223 Rem and 5.56 NATO are that the throat length is longer and the chamber pressure is measured differently.

throat length, c Real Guns

“Throat length” is defined as the distance between the end of the cartridge neck and the point at which the rifling in the barrel engages the bullet. In the above image the throat length is clearly identified as the green markings on the bullet and marked as (a) on the barrel.

A longer throat length will allow you to load a heavier bullet (since length is the only way to increase weight with a fixed diameter projectile and using the same materials), which are more accurate at long distances and such. But with lighter bullets, a longer throat means that the bullet will not engage the rifling as quickly as desired and may lead to concentricity errors as the bullet wobbles off center before hitting the rifling. That’s a bad thing, and negatively impacts accuracy.

A shorter throat length means lighter bullets are more accurate. But if you try to load a longer bullet, the short throat length will push the bullet further into the case which increases chamber pressure and can lead to explosions and other bad things.

So, in short, longer is better. And 5.56 NATO is longer.

Speaking of chamber pressure, that’s the other thing that changed. The NATO maximum chamber pressure is 12% higher than the SAAMI .223 Remington maximum chamber pressure. However, due to the way in which those chamber pressures are measured (NATO measures at the throat, SAAMI does not) the pressures aren’t exactly the same. Nevertheless, the common belief is that 5.56 NATO pressures are higher than .223 Remington pressures.

Due to the difference in throat length and chamber pressure, the conventional wisdom is that .223 Remington ammo is safe to fire through a 5.56 NATO gun, but not necessarily the other way around. But due to the different throat length, the .223 Remington ammunition won’t be as accurate.

There is, however, a compromise. The .223 Wylde chamber that is used in most National Match AR-15 rifles is designed to combine the best of both specifications and work for either caliber. I believe it also has a longer throat than either spec, which means that you can use longer bullets than anything else. But again, longer throat can lead to concentricity issues.

In my experience, after years of not caring and firing both through either barrel, I get the feeling that in the end it really doesn’t matter. Even so, I try to always buy 5.56 NATO barrels.

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    • But only if they are fired from big black scary looking guns. Its those guns that give them the cop seeking armor piercing capabilities.


      • Yes, without the all important bayonet lug and folding stock, the round is merely ‘intermediate powered’ and can only kill Coyotes and the occasional Poodle.

    • All ammunition is made to kill something. There is a myth going around that these rifles are only meant to kill people. The AR15’s that are sold these days make excellent hunting rifles. They are lighter than most standard hunting rifles, making them easier to carry long distances, a plus for olde farts like me. The round is the same if not better than a 30-06 for retaining accuracy beyond 300 yds. I choose this distance because it is an average of distances fired for a long range shoot. The round can bring down bear beyond that. (not to mention deer, feral pigs, boar, elk, moose, etc.) Never shoot one out to 300 miles!? I thank you for clarifying the points about pressures and shapes in the article. I am an expert and gunsmith.

      • .223 is great for smaller game, but nothing large. In PA you cannot use semi automatic rifles to hunt anything, but in quite a few states you cannot use .223 to hunt large game(ie: Deer, etc) because it’s not lethal enough. High powered tiny bullets go right through the damned thing and won’t kill it, only maim it, and when you’re hunting deer you want it to drop dead on the first shot, not run off into the woods and die 2 days later.

        If I wanted to wound and track a bleeding animal I’d go bow hunting.

        • I should go tell that to the two deer I’ve killed with my AR-15. Oopps, I can’t tell them. THEIR DEAD. Dropped in their tracks. They must not have gotten the memo that .223 wouldn’t kill them.

        • A 1-7/8″ Broadhead with a 5 oz arrow behind it is much deadlier to an animal than almost any .30 caliber round. There is a film of Fred Bear shooting a half ton Kodiak bear in Alaska with a 40 lb. recurve back in the sixties. The bear only made it about a hundred feet before it dropped dead. Also the steel points on most modern arrows will penetrate Type IV SAPI plates.

        • Bow & Arrows kill very effectively. Hunting with both I can tell you that just as there is a difference with hunting with a traps, shotguns and high powered rifles, Archery is also different and very lethal. Two of the main differences is the expertise to get close enough to take a shot the other is the intended method for the prey to die. Guns use blunt force, where arrows are intended to “preferably” NOT get the prey’s adrenal glands to ruin the meat and let them wander a short distance, lay down and bleed out. Hence the very important design and selection of e broad heads. If you really like to hunt, then learn both or all methods.

  1. I just got done reading a Hornady reloading manual, I don’t plan on doing my own loading for a while, but it was a nice and easy way to get a better grasp on the differences in ammo’s, barrels, and chambers.

    This general info wasn’t hard to understand or terribly long, the majority of the book is specific info for specific ammo loads

    • I just got done reading a Hornady reloading manual

      You’re more studious than I was. My eyes glazed over by the time I reached .270 Weatherby Magnum.

  2. And if you have the urge to shoot that between-wars custom Brit bolt job you found in Grandad’s gun case, 7×57 is much cheaper than factory .275 Rigby and a lot easier to find.

  3. Does it really matter? My Armalite is chambered for 5.56, will shoot .223, but I can’t find any ammo in either caliber.

    • The .223 and 5.56 gunpowders are also not considered a sexx booster as the Chinese believe Rhino powder is capable of even though gunpowder was invented in China.

      • Why do the Chinese need a sex booster? Aren’t a 1.3 billion Chinese enough for now? IMO, the only powder that the Chinese need is saltpeter, and not for their gunpowder.

    • I’m seeing retail stores here with ammo on the shelves at $0.60-0.75 per round – much better than a month ago when Russian steel was going for $1.00 each. I’ve purchased 1,500 rounds in the past 2-3 weeks at an average of $0.40 per round for steel and $0.50 for Federal LC XM193. Not bargain bin pricing, but not absurdly higher than pre-madness rates. I have another 1,000 Hornady Steel Match on backorder at $0.40 per which is what it retailed for previously. You just have to look and be ready to order when you find what you want at a good price.

  4. My experience with these two cartridges definitely indicates firing 5.56mm ammo in certain .223-chambered rifles can cause problems. I’ve owned two different domestic-made break-open .223 rifles that would not extract or eject empty cases when fired with 5.56mm ammo, and fired cases showed unmistakable signs of high pressure (flattened/cratered primers, with some showing gas blow-back from around the primer cup — on crimped primers!). Older .223 bolt-action rifles displayed terrible accuracy, hard extraction, and occasional punctured primers (with high-pressure gas flowing into/through the action) when fired with 5.56mm ammo.

    I was lucky in my younger and less-enlightened days, and suffered no injuries or significant firearm damage; but, as I now understand the issue, I wouldn’t do today what I did back then. As a matter of fact, to avoid the issue, I try to only buy/use 5.56mm-chambered firearms, and stay away from .223-only guns unless there is a darned good reason to get one.

  5. Nick thank you for the info and just in time. To bad I can’t find either bullets or rifles in ether caliber.

  6. Ives seen both local Prison guards at a local State Prison use Mini-14s and for training they use 5.56mm M-193 ball ammo. No problems with them like ive seen AR-15 users use .223 ammo as well. for semiauto both are fine but for bolt action or single shot be careful!!

    • Depends on the bullet weight, your barrel quality & length, etc.

      People in the accuracy game play with the leade on their loads – a lot. Some benchrest shooters eliminate the issue entirely and seat their pills into the lands, appropriately reducing their powder charges to account for the higher pressures that result.

    • Not, 7.62×51 is safe to shoot in .308 chambers
      and vice versa

      there is only one “but” and that has to do with some semi automatics

      Not a safety issue, but if you shoot a 308, with a heavy pill (and slow powder) it will cause higher pressures at the gasport, and for instance in an M14 it will cause a more violent movement of the Oprod.

      Your gun will not kaboom from it, but your expensive oprod, will wear out a lot faster and potentially bend.
      Chamber pressures should be equal, it’s just that heavy pills with slower burning powders will burn longer and the bullet will accelerate slower, so the pressure is higher towards the gasport as a result. So the oprod is slammed a lot harder into the receiver.

      The whole discussion about 7.62 vs 308 is because a number of fools can’t read pressure specs and their nonsense has been recirculated over the internet by other fools who can’t spot the fault in their reasoning and argumentation.

      The fault being that they compare the 50 000 cups for 7.62 to 62000 psi indicated for 308 and then they say, “hey, that 308 is rated for higher pressure, so you can’t shoot that in a 7.62 rifle.”

      But that is just bullshit, because they are comparing 2 different units of pressure.

      50 000 cups is not the same as 50 000 psi, it’s close to 60 000 psi , close enough to be perfectly safe. And that is what SAAMI says about it as well.

  7. These rounds may or not be “evil” or “powerful”. but here in New Zealand, the hunting guidelines strongly suggest against using anything less than .243 on our big, tough deer.

    Only goats or wallabies are suitable game for the .223/5.56 calibre rounds.

  8. As to “how much pressure is too much pressure?”

    Depends on the gun and cartridge involved, of course, but the SAAMI “rule of thumb” is that modern brass rifle cartridges are designed to contain up to 70,000 PSI reliably – once. That does not mean that a cartridge case that held the dragon in check at 70K PSI is fit to do it again. I, personally, don’t like to push my brass that hard and try to keep my loads down under 60K PSI in non-magnum cartridges so I can reload it. YMMV.

    Being over 60K PSI isn’t a cause for concern – some modern magnum rifle cartridges are over 60K and some are upwards of 65K, so if you have NATO-spec ammo that’s nearing 60K, it’s not an exceptionally high pressure. To give a comparison, older cartridges (eg, the .30-06) used to fall in the 54 to 56K PSI range, and even older cartridges left very generous safety margins – eg, the 7×57 Mauser is down around 48K PSI, as originally loaded.

    That said, WRT the original question about 5.56 vs. 223 Remington pressures: I wouldn’t expect a factory-new 5.56 loaded into a .223 chamber to cause catastrophic failure as pictured above. The pressure excursion if the bullet is seated into the lands would certainly leave some signs on the brass, but I’d expect a less-than-catastrophic indication. I’m not saying it can’t happen, I’m saying that, given my experience, I wouldn’t expect it to happen.

    Whenever one gets a new lot of NATO-spec ammo, especially from off-shore, one should fire a few rounds and examine the brass closely. This is where I think the lack of reloading knowledge is hurting many modern shooters – because they don’t reload, they don’t know how to “read” a cartridge case for signs of excessive pressure. There are several signs you typically observe before case failure that tell you your pressure is “too high:” Primers walking out of their pockets, flattened primers, primers that walked back so hard that you can see transference of machining marks off the bolt face onto the primer cup, hard extraction, bright rings around the case, split case necks, case head flow into machining marks on the bolt face, etc… all are warning you that you have excessive pressure. Pick up some of your brass occasionally and look closely at the case heads and interpret what you’re seeing. If your rifle “sounded funny,” stop and examine the brass and check the bore. If you see signs of excessive pressure, heed these signs and start investigation of the reason(s) why you’re seeing them.

    A catastrophic failure such as the above is usually the result of a squib load lodging a pill in the bore, followed by a non-squib load which caused the barrel and action to come apart. Another cause is some sort of obstruction – eg, a cleaning patch left in the bore, etc. The case has to find a way to vent, and the M-16/AR-16 bolt will tend to channel gas flow down the firing pin hole, which then:

    a) blow off the extractor (as you see above), leaving behind the roll pin that held the extractor in place, which will

    b) blow the right side of the upper outwards as the gas then tries to blow out the side of the bolt, but it will encounter resistance (at least until the aluminum on the upper is pushed out of the way) so the gas pressure then will try to exit around the cam pin… but that’s a limited path, so the pressure…

    c) will continue down the FP channel, exiting the rear of the bolt, where it will both

    d) run up into the gas key, and blowing the gas tube out of the front of the key, which results in more pressure being dumped into the front of the upper (which isn’t vented well) and

    e) dump out the rear of the bolt, down into the lower action, where it has few escape paths without destroying stuff in the way, like blowing out the bottom of the mag, blowing through the trigger/sear area, etc.

    Modern rifles, especially semi-autos, have little to no provision for case failure. eg, Compared to the care with which Mauser designed the Mauser 98 to handle case failures, the M-16/AR-15 makes no real provision for case ruptures or failures. If you have a case failure, your rifle will look very similar to that one pictured above.

    Summary: Learn to read brass.

    • clearly you can all tell this is a fake photo, right? image fab is all the rage now. clue #1 why would the mag well be destroyed like that? clue #2 the bolt looks way to big and where the chunk is missing it looks to be solid.
      it’s a fake image

  9. Ok, so let me ask this. Is there NO difference between 223 & 5.56 brass – can I reload 223 brass with a longer bullet, and 5.56 specs?

    • From my examination, I have not found enough to worry about, unlike 7.62 NATO vs. .308 commercial brass from several years ago.

      Weigh your cases. Weigh a bunch of .223 Rem commercial cases (empty, no primer), then do the same to NATO cases. If they’re the same length (as indicated with a set of calipers) to within a couple thou), and they’re the same weight within a couple of of grains, then they pass the first test. The nominal weight of .223/5.56 cases I see is in the low 90’s of grains – like 91 to 94, with most falling in the 92 to 93 area.

      You can then move on to water capacity. Plug the primer flash hole with something (a toothpick) and pour in some water until you fill the case to the top of the neck. Pour into a measuring container (best use one calibrated in mL) and compare the volume of both cases. If they’re within less than a mL, you’re GTG.

      • So is it that 223 brass tends to be thinner than 5.56? Or is thicker and produces higher pressure with 5.56 bullet and powder specs in it?

        • In the past, 5.56 brass was thicker than .223 – and had (as a result) lower internal volume.

          This resulted in higher pressures.

  10. “In the beginning, there was the .223 Remington cartridge which was a version of the .222 Remington cartridge tweaked to work in the AR-15 design. Then the military took it and jimmied the specifications a little further to make it suit their needs.”

    Factually incorrect. Companies working toward military contracts and under military guidance developed everything involved with the AR-15, including the cartridges. The commercial side derived .223 Remington from some of the early military work with SCHV projectiles.

  11. Good article.

    What I’d like to know is what exactly happened with the picture you posted of the exploded AR. I would bet that it was an incorrect handload/reload cartridge or something like that.

  12. This is probably the best article I have read thus far on the differences between the two rounds. Very informative. The picture says it all.

  13. Does anyone have a considered view on the plausibility of an M4 with a high capacity .223 magazine having been used at the Sandy Hook school, given the overall report of:

    a) the young lad having carried the Bushmaster around with two other hand weapons, and

    b) firing 2-5 rounds into each of 26 victims within 7 minutes and

    c) with no professional training at ranges, only the odd trip to target practice with his mum and written & online manuals, and

    d) the coroner describing “This type of weapon is not, uh … the bullets are designed in such a fashion that the energy—this is very clinical. I shouldn’t be saying this. But the energy is deposited in the tissue so the bullet stays in [the tissue].”

    e) State police Lt Paul Vance said one person suffered an injury and survived, indicating that Lanza was unusually accurate or methodical in his fire – or that most of his victims were shot at point-blank range.

    For those of us with little experience with these rifles, do you have any calm & thoughtful comments on how cohesive these descriptions are please? Or are there important discrepancies on what this weapon and ammo are capable of? Thanks in advance.

    • Corrections and additions to the above posting about the Sandy Hook school incident:

      – the rounds were 556 NATO marked “S&B 60 5.56 X 45”, not .223’s
      – the murder weapon was a Bushmaster XM15-E2S, not an M4 (oops)

      The police report is available at “” and some interesting points from it are summarised below which you guys may have some insights into:

      – 5 rounds escaped Classroom 10 hitting 3 civilian cars parked outside.

      – the shooter delivered all 153 round in 6 minutes before suicide.

      – the shooter was strongly Aspergic, 6ft tall and weighed only 112pounds. He carried in ammo & 3 guns weighing 30½ pounds in total, ie. 27½% of his body weight.

      – of the eight PMAG30’s used, 3 live rounds “fell” out onto the floor, and one more live round “fell” out into his pocket. Is this a common failure ?

      – Each of two classrooms had 2 adults and 16 children. In the first the shooter expends 4 rounds/victim and 1 escapes, then in the second (final) classroom he expends 7 rounds/victim and 11 escape. This could just be nerves, as sixty seconds after the first police car arrives, he commits suicide. The shooter wore earplugs throughout.

      • I’m still not convinced Sandy Hook even happened. There’s just too much evidence out there that contradicts the event. My favorite is the video of one of the “dads” who doesn’t realize he’s being recorded, and he’s all chipper, laughing, excited right before a press conference. As soon as he’s ready to walk to the podium, he immediately gets into character, somber, tries to bring up some tears and a sad face.

        It may have happened. I just don’t put it past our governments to fake something like that. There were only a few people who actually went into the school, and of course even the photos of the school didn’t match other photos. Photos on the school website showed different school colors from those taken by the person inside. Photos outside the school captured by Google Maps didn’t match those captured by helicopters flying over the scene. Helicopter video shows the exact same people walking in circles not much different than the extras in The Truman Show. Helicopter video also showed handicap spaces not maintained, fire exits clearly blocked by boxes and supplies, which supports the reports that the school had been closed several years before. Internet caching didn’t match a school that was being used. There was NO internet activity in the building for several years prior to the event. Facebook pages of some of the victims were amazingly created just a day or two before the event. I’m bored. Do your own research.

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