Home Forums Hunting & Shooting Ask The Gunwriters CUP vs. PSI in 5.56 NATO???

There's a discussion on another forum about pressure differences between the .223 Remington and the 5.56 NATO rounds. From what I've been able to find, the spec is 60k CUP for the 5.56 versus 50k CUP for the .223. Is there a PSI spec for the 5.56? (I understand that the two measuring systems don't correlate well).

The discussion on the other board surrounds the necessity of crimping primers in the 5.56 due to the higher pressure. My own opinion -- speculation really -- is that even at 60k CUP, crimping primers should not be the cause of primers popping out of cases. Insight would be appreciated! Thanks.

60,000 CUP sounds very high to me???


This from Wiki- not sure the original source as I didn't go that far but it's not 60K cup according to them

The piezoelectric sensors or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ. This difference in measurement method accounts for upwards of 137.9 MPa (20,000 psi) difference in pressure measurements. This means the NATO EPVAT maximum service pressure of 430 MPa (62,366 psi) for 5.56mm NATO, is reduced by SAAMI to 379.21 MPa (55,000 psi) for .223 Remington.[7] In contrast to SAAMI, the other main civil standards organization C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56mm NATO.


Sounds like the higher pressure is a result of 3 possible things; thicker brass which means less volume, different place to test the pressure on the brass/chamber, and just the organizations preference and testing protocol for that chambering.

Yeah, I'm guessing (without looking it up) that it's 60,000 psi.

From my previous post: "...even at 60k CUP, crimping primers should not be the cause of primers popping out of cases." Obviously not. This is so poorly worded that even I can't tell what I meant! My hat is off to you guys who deal with this grammar thing day in and day out!

I thought 60k CUP sounded high too. What I was trying to ask above is what the reason for crimping primers in the military ammo is. Is it because of the higher pressure, or is it because they are running this stuff in full-auto mode? Thanks for the replies!

It's not for pressure, but rather the excessive headspace sometimes developed in battlefield rifles and for ultimate reliability in various weapons systems. Even 40,000 CUP will pop a primer out if it isn't supported by the bolt face.

Military ammo has a long history of crimped primers. I'm not sure all of it is spec'ed that way but a lot is.

Is there not a difference in the throating. Without checking reference books, I seem to remember that the 5.56 NATO has a longer throat than the 223 Remington, similar to the Weatherby chamberings. Thus, the bullet has to jump some distance before engaging the rifling and creating a pressure spike.

I don't know where all the BS comes from BUT a crimp won't hold in an overpressure load from blowing the primer. Once brass starts to flow a crimp can't do a damn thing. Argue all you want but if folks believe that a crimp will save it from pressure they are not well educated.

Not directed at anyone. Just venting. Ive heard the argument time and again.

FWIW I've run 60-62 psi loads in teh AR quite a bit. They are very strong guns but an overpressure load flows brass and its all over with. Crimp or not the primer is coming out.

Jeff

And whether the crimp does or doesn't work, it's theoretical intent is as a "safety precaution" to safeguard against the possibility of loose primers ending up under trigger assemblies, thus rendering the weapon immediately useless and/or temporarily inoperable........BTDT many times.

Brass used many times well within accepted pressure levels but especially newer, less used brass pushed towards the upper end pressure limits...........somethin's gotta eventually give. As Jeff said...............the primers are comin' out, crimp or no crimp. That has been my experience and the main reason that I'm not shootin' every second when visiting my varmint haunts. I can oftentimes be found fiddlin' around with tweezers and a small flathead in and around my BM's trigger assembly in a panicked attempt to extract said obstruction to get back into the game as fast as possible.

I've backed down a bit from my "ultimate varmint mistin' load", to try to prolong case life......it has done so, but ever so minimally. It's one of those happy medium things I guess.

Whatever the case, I've become an expert at extracting loose primers from in, around and under trigger assemblies which in truth, isn't the worst practice one could involve himself in. There is some truth in "having too much of a good thing" however.............grin.

That's my understanding -- longer throat and maybe a bit larger diameter in the neck portion of the chamber. The common advice (and I think this comes from SAAMI) is not to run 5.56 ammo in a .223 chamber. There are also some other chamber configurations, such as the Wylde chamber, that are supposed to allow the use of both rounds without the theoretical loss in accuracy you get when running .223 in at 5.56 chamber.

I'm skeptical about the 60k figure being CUP, but I have read that in more than one source, albeit hearsay. I don't know. And, my suspicion is that the reason for the crimped primers has nothing to do with pressures standards either.

There's BS on the internet?? A friend of mine once commented that he doesn't believe anything unless he sees it himself, and even then, he only believes about half of what he sees!

A longer throat would lower pressure and not create a pressure spike. Look at Weatherby chambers.

I don't know the differences in the chambers of the two, but a lot of the military stuff (5.56) is 69 grain or the SS109 stuff which is a long bullet as compared to most of the civilian stuff which usually is 55 grain and under. Federal makes a gold medal match load with a 69 grain Sierra but I'm not really sure why. We used to use it in all the tactical rifles and snipers but switched to a much more frangible bullet. I think Hornady TAP ammo with a poly tipped bullet.

Different organizations use different pressure testing methods, which results in different numbers describing the same event.

According to Wikipedia, NATO specs the 5.56 at 62,366 PSI.

NATO pressure specification

According to another source, NATO does its pressure measurements at the mouth of the cartridge, as CIP does in Europe. This results in a pressure reading different from our SAAMI method.

NATO method

SAAMI specs the 223 at 52,000 CUP and at 55,000 PSI.

All these pressure measurement methods, plus the CIP crusher specs, are readily converted to one another.

I don't have data to create a conversion for NATO specs, but if you want to compare CIP and SAAMI pressure numbers for rifle cartridges, then, to a good approximation:

CIP PSI = 18,419 + .695* SAAMI PSI

CIP CUP = 2428 + .825 * CIP PSI (very precise fit)

CIP PSI = 4772 + 1.067 * SAAMI CUP

This last result is extremely curious. For all practical purposes, CIP PSI is numerically the same as SAAMI CUP, offset by an amount equal to the strength of the brass case.

If you want to assume that NATO is using the CIP method (maybe, maybe not), then 62,366 NATO PSI would be very close to 54,000 SAAMI CUP, vs. the SAAMI 223 spec of 52,000.

Above is not correct. CIP has NEVER measured any pressure at case mouth, with the exception of Rimfire cartridges. (CIP is not only Europe but also some South America)

Nato EPVAT is measuring at case mouth.
CIP and SAAMI are using a gas port (drilled hole) at about 1 inch (SAAMI) from breech face or 25mm (CIP) from breech face.
If the center fire case is shorter, this distance from reech face is shortened.
There is not much difference between 1 inch and 25 mm causing different results.

The maximum average pressure is the same value = 430 MPa (equals 62366 psi)for both Nato EVPAT and CIP.
The SAAMI MAP pressure is set to 55000 psi.

CUP pressures are no longer listed by CIP, because it's no longer allowed to be used.

SAAMI is listing CUP data, because some manufacturers are using vintage equipment. This is no drawback, the CUP and psi data are correlating.

Maybe we will se in some future an equalisation between SAAMI and CIP data for the .223 Rem. I hope so.

VihtaVuori believes otherwise.

Since I also consider H. Broemel to be expert in this area, I am unable to resolve which is correct.

Vithavuori is wrong in it's reloading manual.
Probably the guy who wrote it saw at Lapua's laboratory Nato proof barrels in use and so this faux-pas has happend.

By the way, you can download the whole CIP rules together with all data sheets from CIP's website (also in english language):
http://www.cip-bp.org/index.php?id=tdcc-telechargement

I'll leave the "can you correlate SAAMI psi to SAAMI CUP to CIP psi" question to the smart guys.

But I'll add a data point to the "how does NATO 5.56 pressure spec compare to SAAMI 223 max" question. The US now employs a 5.56 round called MK262 in limited use which propels a 77gr bullet at somewhere around 2900fps. Supposedly because it was loaded to the higher NATO max pressure. Those of us in the Highpower game know that you're doing pretty good with a 77 going 2750 so it was a plausible explanation. I being a skeptic though thought of other possible reasons for the difference such as optimistic reporting in the gun mags, or reporting done with a SAAMI chamber (which does have a significantly shorter throat).

As it turns out, after getting my hands on some of this ammo and running it over the chrono, MK262 does get significantly more speed (pretty much as reported) than my Highpower 77's out of a NATO chamber, so both theories go out the window. It appears to be the real deal as reported! In retrospect, I should have compared strain pressures, but my strain barrel is a Wylde chamber and my main intent was to debunk the supposed velocities out of a NATO chamber, and I only have a very small amount of MK262.

Another reason I suspect the higher NATO pressure is in this case, the same manufacturer also loads a SAAMI spec 77gr round that clocks 2650 to 2700fps.

Now that my curiousity is piqued, I think I'll go buy some factory 223 to compare with my the data I already have generated from M855 NATO spec ammo out of my Wylde strain barrel.

h broemel....

Thank you very kindly for the link. This is information that I have been looking for. That part of the site seems to be in the process of being revised. If you have this document in English, would you please email it to me? My email is denton(a)pmg.cc. (using (a) instead of the usual sign, so spammer's robots don't find me).

Mr Bramwell,
I think this is something you've been looking for;
http://en.wikipedia.org/wiki/NATO_EPVAT_testing

Make sure to download the references and check out the links to related articles.

ChrisF... Thanks!