.223 vs 5.56 NATO pressure difference discussion

[big al hunter]

A couple of recent threads have left me wondering enough that I went back to review some research I did some time ago. I spent a fair amount of time researching the pressure ratings of .223 and 5.56. It makes no sense to me that very near to exactly the same cartridges would have a 7,000 psi difference in pressure. My studies left me thinking that an awfully large percentage of the gun world is confused.

One of the articles I read that sparked my search for more knowledge was from a gun magazine. I can't remember now which one. The author was curious about the pressure difference between 223 and 5.56. He put pressure reading equipment on production rifles with both chambers and fired both loads in all of the rifles. Some of the 223 loads had higher pressure than 5.56 in the same chamber. That got me thinking. Why....


My research found that the military has 2 different specifications for 5.56, that have different pressure limits. The limit many people quote for 5.56 is 62,000 psi. This appears to be due to the NATO EPVAT specification for pressure of 62,366 psi (430 mpa). The second specification is US military SCATP with a max pressure of 55,114 psi (380 mpa). Which is the commonly used pressure for .223.

When I found these specifications it made me even more curious. I used some Google Fu and found that the 2 systems are used because in the US ammunition is produced using SAAMI specified testing method. Which happens to match SCATP specifications. Foreign military ammunition manufacturers do not use SAAMI specified pressure testing. NATO created the EPVAT specification for their use. The 2 specifications test the pressure at different locations with different protocols. Thereby giving us 2 different pressure numbers for the same cartridge/load.

I know there is a difference in the 5.56 chamber that allows use of heavy for caliber bullets. And that using 5.56 ammo in .223 chambers could cause issues with pressure. Possibly making it dangerous. But the standard test pressure is the same for .223 and 5.56 when using the same measurement specification.

My questions:

1. Why is this not common knowledge?

2. Do your loading manuals show pressure for loads tested as 5.56?

Of the 3 loading manuals I have, none show a pressure for 5.56. Hornady 9th edition has 3 sections, 2 for 223 and one for 5.56. But no pressure is listed. Nosler Reloading Guide 8 shows both cartridges, but no pressure. Lyman 49th shows pressure in CUP (different discussion) but no section on 5.56.

[mehavey]

I give it as my "fixed opinion" (USMA take note) that the effective pressure
differences between 5.56 and 223 is for all practical puposes -- nil.

Different throat/bullet fit is a different matter -- depending

[JohnKSa]

The reason that this isn't made into a huge safety issue is that the mismatch isn't a big one. With no other contributing factors it's not really likely to cause problems. With other contributing factors (such as high heat) it may cause primers to pop. I seriously doubt that any gun has ever been seriously damaged by the mismatch--of course that's not the same thing as saying it doesn't cause problems.

So lots of people get away with it. Also, lots of people think that "It didn't happen to me=it can't ever happen." and other people seem to agree with them. And some people who don't get away with it, attribute the problems they see to other factors such as poor ammo quality or freak occurrences.

And there's the other complication that the different pressure standards involve different pressure measurement techniques which can result in slightly different readings. That clouds the issue somewhat. However, to clarify things a bit, Black Hills has flat out stated in a press release they made upon introducing a new 5.56 loading that they load at least some of their 5.56 ammo to levels above .223 and warn against using it in .223 firearms. I can't think of a reason for them to lie about that.

But the standard test pressure is the same for .223 and 5.56 when using the same measurement specification.

Both Federal/ATK and Black Hills have stated categorically and on the record that 5.56 is loaded to higher pressures than .223. As far as I can tell, the "confusion" on this topic is more about what people want to believe conflicting with reality than it is about truly not being able to find the truth stated clearly.

Here's the quote from Black Hills:

Sept/October American Handgunner
Black Hills Ammo-Family Values--No Compromise Quality
by Wayne Van Zwoll

"We do distinguish between .223 and 5.56 ammunition," emphasized Mike Wright. "Per SAAMI specs, the 5.56 NATO is loaded to higher pressures. You may see identical velocities in our data. That's because we chronograph 5.56 ammo from 20" barrels, .223 from 24". BHA does not recommend use of 5.56 ammo in barrels with .223 chambers.

[pblanc]

The fact that NATO ammo is loaded to the EPVAT and US made ammo is loaded to the SAAMI does confuse the issue since the two pressure limits cannot be directly compared. But when using the same pressure measuring protocol, 5.56 ammunition may be loaded to higher pressure, but most often it is not.

I have looked at garden variety 55 grain FMJ ammunition produced by a number of manufacturers who market both .223 Rem and 5.56 x 45 ammunition with the same projectiles and the data presented for muzzle velocity for the two is often identical. My very unscientific observations shooting both varieties in the same rifle indicate that there is no real difference in drop between the two.

It would be a bad idea to shoot some 5.56 ammunition in a barrel with a 223 Rem chamber. An example would be the M855 green tip NATO round. The chamber on military rifles initially identical to 223 Rem chambers had to be modified to lengthen the leade when NATO adopted that round. That is because the ogive on the M855 round is farther from the case head than for the M193 cartridge. Using those rounds in a 223 chamber could result in bullet set back and an over-pressure situation.

[Unclenick]

I've done more research on this and found not all the pressure differences are instrumentation artifacts, but some are and some of it is quite confusing.

When the U.S. adopted the M193, in its initially developed form it was in a SAAMI-like chamber and measured a maximum average peak pressure of 52,000 CUP at the chamber, which is how SAAMI came to adopt that number for 223 Rem originally. Then over the next couple of years, the military chamber was refined to add some freebore and taper the back end a little more and the same loads then shot to 50,000 CUP and that became the military's maximum pressure for that round. This is handy information because it tells you a 5.56 chamber will likely run about 4% lower pressure than the same round in a SAAMI chamber will. But that is just an average. Other factors can affect it. But I'll pretend the average is right for this post.

When SAAMI used 223 reference loads developed by copper crusher protocols to calibrate conformal transducers, which don't measure pressure through a hole in the case, but rather over top of the case, the readings they got indicated the transducers would read 55,000 psi at maximum. Thus, 52,000 CUP and 55,000 psi became the SAAMI MAP (Maximum Average (peak) Pressure) limits.

It was when the U.S. Military tried to get the M193 round adopted by NATO that the first real difference in pressure occurred. NATO felt the M193 didn't penetrate helmets at enough range to qualify, so they developed the SS109 round to use instead. This round reads 55,000 CUP on a military copper crusher or 110% of what the M193 does in the same test rig. The U.S. started to make the M855 round as our NATO compatible version of the SS109, loading it to that 10% higher copper crusher reading.

If you assume the same ratio of pressures going from copper crusher to SAAMI conformal transducer, you would expect M855 to measure 58,173 psi, and, indeed, some declassified information indicates that Federal and other ammunition contractors outside Lake City use a number very close to that to produce contract M855. The NATO EPVAT system, which uses a Kistler channel transducer sampling gas off a hole in the chamber just beyond the case mouth, measures these same cartridges as having a maximum average pressure of 4300 bar (62,366 psi), or a 7.2% higher magnitude than the conformal transducer calibrated with SAAMI reference loads does. Europe decided to adopt this pressure for the 223 Remington as well as for the NATO SS109. Based on the ratios, if they had copied the SAAMI copper crusher MAP, the channel transducer would be expected to read about 55,750 psi or about 3844 bar before rounding.

But then comes M855A1. This is the new Black Hills-developed "green" round being produced with a lead-free bullet. It is loaded to a still higher pressure using a faster powder to cut down on muzzle flash in short-barreled weapons. I don't have a pressure number for it at this point, but reports are that it erodes M4/M16 barrels faster. I would say that's probably finally getting to pressure I wouldn't want to use in a 223 Rem chambered rifle. It is this round that Black Hills can indisputably state is higher in pressure than 223. And if that higher pressure is multiplied by 1.04 to take the smaller 223 minimum chamber size into account, it could be a primer piercer and the source of pretty short case life.

Several things are apparent from the above. If I fire an M855 round in a 223 Chambered conformal transducer pressure and velocity gun, it is likely to produce a reading of about 58,200×1.04 or 60,500 psi after rounding. It's certainly a bigger number than the 55,000 psi that is currently SAAMI MAP, but it's still not a high number for a modern bolt-action rifle, especially not when you consider the little cartridge has thicker chamber steel around it and a smaller head area supplying bolt thrust than a 308 Win at 62,000 psi does. Additionally, the Europeans load 223 Remington to about 62,400 psi (after rounding) without having problems. The AR can digest it just fine, as it can actually digest the hotter M855A1, provided you are willing to accept faster throat errosion.

So, is there anything to concern yourself with about firing 5.56 NATO in a 223 Chamber? As long as the bullet doesn't jam the lands, I'd say no, with the single exception of the Black Hills M855A1.

So then we come to Western powder's list of 62,366 psi. I called them and asked them to confirm they were measuring it in a pressure transducer. Well, if that transducer is calibrated by copper crusher-compatible reference loads, then my expectation would be that they are producing a load that would read about 66,850 in a CIP channel transducer (note that the CIP uses the NATO pressure number, but uses channel transducers that sample gas through a hole drilled in the case instead of at the case mouth like NATO's EPVAT protocol does).

The bottom line, to my thinking, is to work up an accuracy load starting with SAAMI 223 data. If you want to head toward NATO pressure, multiplying the maximum load by about 1.017 will get you close to a load reflecting 58,200 psi by the conformal transducer with a number of commonly used powders. If you want to go still higher, take a look at Western's loads, but I suspect they are too high. They claim not and that they match the CIP readings, but I don't get that from the ratio of the charges which are more than 1.03 times their SAAMI pressure loads.

A final note: Realize that we are talking here about maximum average peak pressures for a sampling of ten rounds. It is not a required pressure, but an upper limit. To comply with NATO standards for both functional and ballistic compatibility, the military has to meet a minimum gas port pressure and a narrower velocity range than SAAMI calls for (to assure ballistic compatibility, meaning not only impact energy will be maintained but that standard sight graduations will be valid). What are the odds a charge will just happen to hit the gas port pressure and velocity numbers at the same time that it just exactly reaches maximum average pressure? Small. Many lots of powder will hit the velocity and gas port pressure numbers with a peak pressure well below that limit and do. Mix in individual gun variables, and I think this is why you see things like that test showing no difference in produced pressure. There could be a difference with some lots of some loads (especially M855AA1), but it isn't necessarily going to happen. Nobody forces the military or SAAMI to reach maximum pressure numbers and they usually don't. Those are just limits that keep too fast a powder from being used to reach the velocity spec.

[mehavey]

See also:
https://www.luckygunner.com/labs/5-56-vs-223/
actual testing... as well

[Unclenick]

Yep. That's the article I've seen before, and I assume it's the one the OP refered to. I don't recall coming across another.

[big al hunter]

Yes mehavey, that is the article that sparked my curiosity.

In my studies I also came across an old military specification for the SCATP protocol that specifically mentioned that it was designed to allow American manufacturers to test ammunition for the military during time of war situations. And that the SCATP protocol was intended to allow them to test with the SAAMI standard equipment they already had, so they wouldn't have to re tool and change to the equipment needed for the EPVAT protocol. Therefore there are 2 different max average pressure specifications for the same ammunition, but for different measurement systems.

My understanding is that if you tested the same lot of ammunition on both systems, and that the lot happened to be at max average pressure for one protocol, it would also be very close to max pressure using the other protocol.

Unclenick has included some information about the M193 ammunition. It was my understanding that this ammunition was the reason for extending the freebore. This reducing the pressure back to the previously set pressure. I never found anything that raised the max pressure to allow for the M193. Do you have any documentation for this that I could look up? You got my curiosity started again....

Given my assumption that both systems would measure the same ammo differently, wouldn't the Black Hills ammo that is tested to be over SAAMI 223 spec also be over NATO EPVAT?

I agree that it likely won't damage a bolt action rifle to fire 5.56 in a 223 chamber. The difference between the two systems of measurement are small. I have tried to load some 223 for my Savage Edge with the ogive at .02 off the lands. Couldn't seat the 55 grain bullet far enough out to contact the lands to even get a measurement. I suspect it is cut to a 5.56 chamber and stamped 223.

[Unclenick]

In reverse order,

The trick with short bullets is to set them into the throat and hold them there with a pencil or dowel rod or whatever fits. Then set a solid rod into the muzzle to stop on the bullet tip. Mark the rod with a pencil if you are not too precise and with a single-edge razor blade laid flat across the muzzle. Remove the pencil and knock the bullet out. Close the bolt and let the rod slide in to rest on the bolt face while you mark the rod again. The distance between the two marks is what the COL would have to be to contact the lands with that bullet. Subtract the length from the bullet tip to where the rifling scuffs are and subtract that from the first measurement to know where the throat is.

The Black Hills is definitely over the established pressure spec. This will require adoption by NATO to accommodate the numbers if that is ever done. I don't know that they adopt every round we develop, but given the European penchant for embracing anything "green", I'd be surprised if they didn't show interest.

You are correct that the M193 was the reason for the chamber changes. The actual sequence of events and cartridge naming is way more complicated than my oversimplified description. The original military cartridge development started out as the 222 Special made by Remington at the request of Eugene Stoner to get enough powder space to hit his target velocity of 3300 fps with a 55-grain bullet. That was in 1957. It took both the case change and a switch from the generally available IMR4198 and 3031 to IMR4475 (never commercialized for handloaders) to get there. In '59, they renamed the cartridge 223 Remington to avoid confusion, but it was still basically a development cartridge for military equipment. It was during that period that the pressure of 52,000 psi by copper crusher (the military's terminology; CUP to SAAMI) resulted from the velocity work. It wasn't until 1962 that Remington decided to recoup some of its investment by commercializing the cartridge and rifles for it and submitted it to SAAMI with that 52,000 CUP pressure and the chamber dimensions worked out up to that point. The military didn't officially adopt the round until the following year, which is when the M193 designation ocurred. They were still doing reliability improvement work at that time and into 1964, and in that period is when the chamber changes were made and the pressure brought down, as near as I can discern, all after SAAMI standardization.

I expect reliable feeding was a motivator as well as the rate of full-auto fire and how hot the barrel got in that mode, but I am speculating here. I just recall running into a description of the changes happening within that two-year period after the SAAMI standard for the cartridge was in place. I'll have to see if I can dig up a reference, but my spidey sense says it was in a general description of the development of the cartridge and Armalite design and not a declassified military document. So, when I say the 223 Remington was adopted from the military cartridge, it is more specifically accurate to say it was adopted at a stage of development of what became M193 in the end and before all development changes were completed.

[big al hunter]

. The trick with short bullets is to set them into the throat and hold them there with a pencil or dowel rod or whatever fits

I usually go with a spent casing that was fired in the rifle and bend the case mouth slightly. Then color a bullet with marker and slide it into the case just enough to hold it there. Then I put it in the chamber. It pushes the bullet into the lands and causes a mark to show on the bullet from both the case and the lands. I then measure the scuff as you described it to obtain a starting length. This gun didn't contact the lands at all. I could put the bullet in the case about 1/16th inch and not contact the lands. Haven't tried a heavier bullet yet. Might give your dowel trick a try first. Thanks

[Marco Califo]

Western Powders (Accurate, Ramshot) publish different pressure data for 223 and 556. The 556 data being faster/hotter. I am not saying they are correct, but they do publish data that says that.
https://www.accuratepowder.com/load-data/

[mehavey]

I am very concerned that like the great CUP/PSI conversion many years back in which people/companies translated one unit to the other w/o any appreciation that they were significantly different ... we have AGAIN erroneously translated a SAAMi-technique measurement of "psi" pressure back & forth with the CIP "psi" measurement process.

They can (and do) produce two different numbers for the same cartridge.
https://en.wikipedia.org/wiki/Small_...essure_testing

Uncle Nick has written extensively on this, multiple forms, multiple years.


.

[big al hunter]

.I am very concerned that like the great CUP/PSI conversion many years back in which people/companies translated one unit to the other w/o any appreciation that they were significantly different ... that we have AGAIN translated a SAAMi-technique measurement of "psi" pressure back & forth with the CIP "psi" measurement process.

I too worry that gun rag writers are running off at the mouth with numbers that are flat out wrong. Then it gets repeated as gospel....the writers are usually graduates of writing classes that have an interest in guns, rather than science majors that have an interest in writing. I admit that I once believed every word I saw in print if it was behind the cover of a hunting magazine.

[44 AMP]

Color me one of the people who gets confused by the whole this pressure vs that pressure thing, one system gives X and another says Y with the same thing, yes??

Aren't we arguing..er..pardon, discussing whether we are doing 61mph or 100km? And some people think 100 is dangerously fast???

I don't loose any sleep over it, even is there is a 10% or even more difference in the actual pressures (not just the numbers resulting from the system of measurement used) the rifles are proofed to considerably higher, so I don't see a glaring safety issue.

if cases aren't doing what they do when pressure is too high (swell too much, stick, etc) then I'd think pressure isn't "too high", so you're not "going too fast, no matter what the speedometer reads"...

I wouldn't get too worked up about hitting a given pressure spec exactly, EVERYTHING has +/- tolerances, so you're going to get that higher then spec round, and lower than spec round every so often. The guns seem not to care, why do we??
or rather, you?

[mehavey]

If you go much over 55ksi in the 223/556 case, primer pockets rapidly turn into squished doughnut holes in fairly short order.

My rule of thumb: ejector extrusion is sign of 75 and up. Until then few immediate signs.

[Unclenick]

...and even hoping for a sign to give you that much pressure indication accuracy is too optimistic. As you might guess from this list of Vicker's hardness test results, from Hornady to LC, the pressure that causes ejector tunnel extrusion is going to be quite different. And this was all new and never-fired brass; head surfaces gradually harden with enough firings.

[big al hunter]

. Color me one of the people who gets confused by the whole this pressure vs that pressure thing, one system gives X and another says Y with the same thing, yes??

Yes exactly.

Aren't we arguing..er..pardon, discussing whether we are doing 61mph or 100km? And some people think 100 is dangerously fast???

Yes we are doing both 61 mph and 100 kph....in a friendly discussion.

.I wouldn't get too worked up about hitting a given pressure spec exactly, EVERYTHING has +/- tolerances, so you're going to get that higher then spec round, and lower than spec round every so often. The guns seem not to care, why do we??
or rather, you?

I'm not worked up about it. I regularly shoot 5.56 ammo in my .223, because I am running at both 61 mph and 100 kph. Especially with the tolerances and amount of difference between the 2. My hope was that this thread would help all the folks that are lost in the confusion. All too often I hear "don't shoot 5.56 because it is loaded to higher pressure". Not so much....

If this level of misunderstanding is so common, what else is out there that is "common knowledge" that is a little off, or way off? Probably shouldn't ask that question lots of stuff out there

[mehavey]

what else is out there that is "common knowledge" that is a little off, or way off?

The Great Lead Alloy Hardness Formula caper as a function of % tin.
Binary lead/tin stays relatively SOFT (9 and under) all the way up to 50-50 bar solder.
http://castboolits.gunloads.com/show...=1#post4437398

[Bart B.]

223 Rem SAAMI specs state maximum sample pressure is 58,500 psi, average is 55,000 psi and proof loads are 78,500 psi. Rifles must not sustain any damage with either one.

Interesting article....

https://www.primalrights.com/library...nding-pressure

[Unclenick]

The standard sets 55,000 psi as the limit of the average of a single ten-round sample of newly assembled ammunition. The MPLM (Maximum Probable Lot Mean) is 56,400 psi and is how high additonal random samples of ten from the same lot are allowed to average 97.5% of the time while the lot is still new. The MPSM (Maximum Probably Sample Mean) is 58,500 psi and is how high additional samples taken over the years as the lot ages are allowed to average. The MEV (Maximum Extreme Variation) is 11,300 psi for the .223 Rem and is the limit for how much the highest pressure in a ten-round average can be bigger than the lowest pressure among the ten. Proof loads are allow a range of 73,500 psi to 81,500 psi. This is because no two guns will achieve the exact same pressure with a standard proof load.

[T. O'Heir]

SAAMI Max pressure for the .223 is 55,000 PSI, not 62,000 PSI.
The two cases are identical. The biggest difference is that 5.56NATO is used in MG's. And the Remington very rarely.
EPVAT(Electronic Pressure Velocity and Action Time) and SCATP are just different methods of measuring. SCATP is basically the same as SAAMI. Difference between 'em is 114 PSI.
"...the great CUP/PSI conversion..." There is no mathematical way of doing that. They're different methods of physically taking the pressure.
Go here, about half way down for a good explanation.
https://ultimatereloader.com/2018/08...cts-and-myths/

[Unclenick]

T. O'Heir,

SAAMI is 55,000 psi, but CIP and EPVAT use 4300 bar (62,366 psi); but determined by channel transducer systems where we use the conformal transducer system.

[mehavey]

Just in case you thought it was safe to go back into the shower....
https://ballistictools.com/articles/...-and-facts.php

[big al hunter]

. EPVAT(Electronic Pressure Velocity and Action Time) and SCATP are just different methods of measuring. SCATP is basically the same as SAAMI. Difference between 'em is 114 PSI.
"...the great CUP/PSI conversion..." There is no mathematical way of doing that. They're different methods of physically taking the pressure.

T O'Heir. Go back and re-read the article you linked. It explains that SCATP and SAAMI use the same protocol for testing and therefore have very similar results from the same lot of ammunition tested. The difference shows up with EPVAT protocol using different types of equipment, at a different location on the chamber. Therefore it gives a higher pressure reading of 62,000 PSI for the same lot of ammunition that tested to 55,000 with SCATP or SAAMI protocol.

The key is that both methods (SCATP & EPVAT) are used to test the same cartridge....5.56 NATO. If there is to be true interchangeable arms and ammunition between NATO nation's, then all ammunition and firearms for that ammunition must be assembled to the same standard. Because the American producers don't use CIP/EPVAT equipment for testing the US military created SCATP for domestic ammunition manufacturers.

[big al hunter]

Thanks for the link mehavey. Good article.