.223 vs 5.56 pressure barrel
A close-up of a pressure barrel in place, ready to start recording the .223 vs 5.56 events of the day.


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: Measuring Pressure

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.

.223 vs 5.56 chamber comparison
A .223 vs 5.56 chamber comparison.

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 dependant 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.


  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.