An incorrect barrel - 2.23 vs 5.56
Here we see the chips from a 5.56-marked barrel that obviously wasn’t.


Editor’s Note: This article on .223 vs 5.56 comparisons is an excerpt from Gun Digest 2013, the world’s greatest gun book.

.223 vs 5.56: Things Get Ugly

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.

.223 vs 5.56 steel removal in chamber
On a good .223 barrel, the reamer will only remove steel in the neck and throat area. It stops when it is done.

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.


  1. In response to Chick, yes- the longer throat does make a difference in accuracy potential If you are not a reloader, and use only commercial/military loaded ammo. The exterior cartridge case dimensions are exactly the same. Over -all length (OAL) may not be.That is determined by bullet weight! The original cartridge started out with a 52 grn bullet.This is when Remmington who had invested much in the way of the development of the finalized cartridge jumped on the commercial value as a civilian product ,had it SAAMI certified (5.56 x45Nato is not!). Hence …Ta.. Da! =.223 Remington.The only difference as you see, is in the area of the chamber in front of the case mouth.Military bullets weigh upwards of 80 grs. . Now as too leade,/Throat.Heavier bullets require longer throats,lest the bullet jambs into the rifleing .Now you talk about pressure spikes! Yikes!!

  2. If I found that the manufacturer of my AR type rifle wasn’t building a rifle that was capable of firing mil-spec ammo, not only would I get rid of the gun, but I’d make sure everybody at the local gun shops and on social media knew about it. It’s a long article for saying “don’t put hot loads in a weapon that’s not designed for it”.

  3. I am no expert but have had past opportunity to touch up-close those early M16 rifles. Between the M14 and the M16 the first noticeable difference is the very much reduced recoil/total weight of the M16/ammo. The M14 after 200 rounds in less than 1 hour the temptation to hold the butt away from the shoulder was nearly impossible to avoid. Today’s CQB (close quarter battle) tight-urban open-terrain evolution has several needs that one rifle just does not fit unless there is armored units alongside. ROE’s can significantly reduce any weapon system combination. This is a sin of ignorance. The current M4 variant is in a good shooters hand effective at 400-600M soft target, but best at the 0-350m. CQB is in part a product of WW2 and Offense Assault needs. Armored and Mechanized rapid assault requires somewhere along that line of operation infantry support at very close distances relative to the effective range of the armored-biggest gun.

    WW2 U.S. right-Flank Army ‘breakout’ occuring any number of times, a very popular weapon was the M-2 auto select carbines that the crew of M119(?) carried.

    IWS a derogatory term with the I-idiot and the S-stars knew very well, 1966, that units, some, assigned were not completing the mandatory repetitive weapon qualifications. Oh, the paper work showed fine-and dandy good work. One such solution was on a given day 6 shooters fire adequate rounds for each assign person/weapon and bag the empties and turn in to the IWS operatives ‘see, we have done our duty’. After 1000 Rd’s 7.62×51/M14 in one shoot, no more shooting for a week, maybe a month, recoil injury. The M16, also shot at these events would only get hot, sometimes ruining a barrel. The guy with the bum shoulder ended up loading Mags M14/tripod/auto.

    The war fighting evolution continues today. The German ARMP43 or variants began the Assault Rife evolution. A copy cat in the AK47 after WW2. The original Garand M1 development model had a banana magazine(30rn?) but IWS (maybe a bureaucrat) altered that notion and via another competing New-rifle that used the metallic Clip now famous in-sound M1 when the last round ejected. The term – ‘How many Clips you have left’vs magazine of today. BAR’ gunners called the bullet holder?

    In my opinion, which may not be much, the M16 development came about because of NATO needs and what would be a defensive CQB-urban envirnment. Protecting available defensive armor units from an invading offensive Warsaw PAC army equipped with the now famous AK47. Several of the then NATO allies had or were developing their own assualt-rife to fire the 7.62. The AR10, rejected by AMC, variant saw action in a foreign army before the U.S. completed M14 production. The turning point has to do with 1960-63 Air-Mobil attack functions – logistical considerations. Any number of M1 trained soldiers hated the 5.56 for most all of the obvious reasons. The most distractible complaint was that the M16 was worthless at dismounted drill. Worthless, they said.

    WW2/Korean vets mostly occupied any place of importance of the Cold War in the US military 1956-66. The last measurable front line offense vs defensive war was coming to an end. Gulf War 1991 was the ultimate test to this Defensive NATO vs Warsaw PAC capability in the Big D transitioning into the Offensive war that wins any war. Logistics win wars. Tenacity wins battles. A vet of Panama 82nd/Regiment jumped with twice the normal ammo for their M16’s. This was not possible at weight restricting 7.62. Thinking Logistics. The USSR converted over to a lower recoiled/weight 22cal AK74. Today’s soldier carrying an M4 is much better trained and skilled at most every thing war than in 1956. NATO EU 1965 was a cannon fodder strategy first hours-days.

  4. It’s funny, but I feel kinda stupid because it seems to me that what the military was looking for, and ended up with, was something very much like the carbine m-1. Light, maneuverable, moderate power cartridge, full-auto and hi-capacity magazines. Plus we had a s___load of them on hand. A little re-fitting and you never know……

  5. There is another element to all this. Back in the late 1960s I was working with weapons designers at Picatinny Arsenal. I remember having a fairly long lunch hour discussion with a man from Frankfort Arsenal. He described an outline of small arms development as follows. There was a desire to produce a weapon for ‘foreign national’ that was smaller and lighter than those carried by the folks in Europe and North America. (Like those folks in south east asia perhaps. It needn’t be said that the additional sales would be good for the arms industry.) There were other assumptions such as wounding the enemy was better than killing. In addition it was well known that long thin rounds become unstable when they pass into a new medium such as air to flesh and they are apt to tumble producing a nastier, but not necessarily a killing, wound. A ball for example just blasts on as far as it can. A piece of coat hanger wire will fly like an arrow and then will bend and tumble when it hits. The extreme military ’round’ in this regard was the flechette. There were even some experiments with flechette rockets that could be launched from within a soda straw or a cigarette, the filter protecting the person firing it from the back blast, but I digress. In any case the point was the ‘new’ military round was designed to be a maiming and wounding round and as such I think it is a bad choice for the military in situations where there is no enemy that will tie up its forces caring for wounded.