223 vs 5.56

[JohnKSa]

Yes, explain it to me. What causes a secondary pressure spike?

If you don't understand the article, why did you provide it as a source to support your claims?

Ok, whatever. Quote the specific part of the article you don't understand and I'll explain it for you.

Can you agree to the following two sentences?

It is easy to blame the mismatch as it is the "usual suspect" but if that same lot of ammo produces the same symptoms in a non-mismatched chamber, then it isn't the chamber, it is the ammo. And if you do have a mismatch, but don't bother to analyze the brass, you won't have any idea if it was an out of spec round.

It is certainly very likely that if a particular lot of ammunition causes the same problems in both .223 and 5.56 chambers that the problem is the ammunition and not a mismatch issue. Occam's Razor says that in the absence of other information, one could reasonably assume the ammunition is the cause.

It is also true that the one can not automatically assume that ANY issue that occurs during an apparent ammo/chamber mismatch MUST be due to the mismatch. It could be the result of the mismatch, or it could be due to some other unrelated issue (like an overcharge, a bore obstruction, etc.) that is causing the higher than normal pressure.

Nothing I've posted on this thread could be, or should be, construed to contradict either of those two statements.

What I have said is that IF there is a mismatch, and one is experiencing pressure-related issues, one can not and should not discount the mismatch as a likely cause.

 

[Jimro]

JohnSKa,

This image, from https://www.shootingsoftware.com/barrel.htm labeled 55 grain Winchester, is what I don't understand.

How can the secondary pressure spike happen when the bullet is more than halfway down the bore, if it is a chamber mismatch issue.

Jimro

 

[JohnKSa]

How can the secondary pressure spike happen when the bullet is more than halfway down the bore, if it is a chamber mismatch issue.

Whatever in the world gave you the idea that a secondary pressure spike could possibly be the result of a chamber mismatch issue?

Did you actually read the article at all? Where in the article did you find anything that so much as hinted that secondary pressure spikes are related to chamber mismatch issues?

 

[Metal god]

I think the point is/was the second pressure spike is what can cause the damage . If that is in fact the case . I think goes back to how do you really know what caused the high pressure/popped primer/stuck case/what ever else the pressure sign was ? with out all the data . Just because the chamber is mismatched that does not mean that was the cause . Kinda like your stove idea . Just because the kid comes out of the kitchen with a burn on his arm does not mean it came from the stove . could have been the toaster or the lighter you some times use to light the stove . How about something that just came out of the oven and is no longer in or on the stove . Until you know all the data of what happened in the kitchen you really have no idea how they got burnt

FWIW when I jump into threads and write stuff like this they almost always get closed . Lets cross are fingers on this one

 

[Brian Pfleuger]

Honestly haven't read through the whole thread but this thread over at M4 should pretty well settle the questions of what exactly the differences are between .223Rem and 5.56NATO.
http://www.m4carbine.net/showthread.php?55149-5-56mm-NATO-versus-223-Remington-Chamber-Differences

The problems that those differences *could* cause should be self-explanatory.

 

[thallub]

Few shooters of AR-15 rifles know what kind of chamber their gun has unless they are machinists/gunsmiths who reamed the chamber using the proper reamer. AR-15 rifles have sold like hotcakes. Many have been chambered by low wage folks with same loving care they used when flipping burgers and/or hotcakes.

Popped primers have gone from a rather rare event to an epidemic.

Lucky Gunner tested M855 ammo in two AR-15 rifles and a bolt action .223 rifle:

However, I was surprised about the results I saw. There was no dangerous spike in pressure from firing 5.56mm ammo in a .223 chamber. While pressures were elevated in comparison to one of the 5.56mm barrels, they were slightly below the other 5.56mm barrel.

This unexpected difference was also borne out by chronograph measurements, which showed that the Weatherby Vanguard Compact bolt action rifle fired the XM855 ammunition (all of which was from the same lot) at a velocity which was higher than the lower pressure 5.56 barrel, but lower than the higher pressure 5.56 barrel. It’s true that the .223 barrel showed about 5% more pressure for less than 1% more velocity than the lower-pressure/lower-velocity 5.56 barrel, but the difference was not astoundingly large, as some might have expected.

In fact, the higher pressure 5.56 barrel – which was in the same condition as and made on the same production line by the same manufacturer as the lower pressure 5.56 barrel – was putting out XM855 at an average velocity within spitting distance of 3200 feet per second.[/QUOTE]

http://www.luckygunner.com/labs/5-56-vs-223/

For some reason or the other, 5.56mm M16/AR-15 rifles experienced a very few popped primers almost since day one.

 

[Jimro]

JohnSKa,

This link is to a photo of seating depth to pressure that the Handloading, Reloading, and Bullet Casting moderator Unclenick. While the 30-06 is not the 223/5.56 the pressure curve will be similar. Since the only difference we've been talking about is the distance to the lands between 223 and 5.56 chambers, any pressure difference resulting from the mismatch should show up in the primary pressure curve.

According to forum rules #5 dated 9 August 2010 since I don't own this picture my option is to create a hyperlink to it.

http://i2.photobucket.com/albums/y17/steve4102/seatingdepthvpressure.jpg

But note the pressure difference between a quarter inch of jump to the lands and zero jump to the lands. Roughly 6k psi difference, which is in line with the pressure increases "not being dramatic" as we've already agreed upon. Generally Quickload will indicate a 7,200 psi increase in pressure due to a bullet being jammed into the lands. This is inline with what we've seen from the chamber mismatch pressure traces.

But that still doesn't give us enough pressure to cause a popped primer or locked up action, at least not on its own.

I think that you would need a delayed ignition (not uncommon with WC series ball powders) that would really jam a bullet into the rifling prior to the powder catching to get a serious primary pressure spike to see over pressure signs. Alternately a secondary pressure spike would easily do it.

So you see with a delayed ignition problem, the longer freebore and shallower leade angle of a 5.56 chamber could avoid that pressure increase in the primary pressure spike. Alternately there is no chamber that can mitigate a secondary pressure spike, which seems to me to be the most likely suspect for 5.56 ammo issues in 5.56 chambers.

Armalite has this to say about ammunition:
The first few rounds of ALL ammunition, from whatever source or lot, should be checked for signs of pressure or any other defect before firing large quantities. If you have a problem, you can generally bet that the ammunition meets neither SAAMI nor NATO specifications.

Remember I'm not advocating for people to mismatch their ammo and their chamber. If you have a bad lot of ammo, having a 5.56 chamber can mitigate a delayed ignition issue caused by the primer/powder interaction. If there is a secondary pressure spike due to powder issues, then it doesn't seem matter what chamber you have.

Jimro

 

[JohnKSa]

I think the point is/was the second pressure spike is what can cause the damage.

Sure, it COULD. It could also be a number of other issues which can cause pressures to rise above normal.

However, the existence of other issues that can cause abnormally high pressures is not in debate. Everyone acknowledges that they exist and that they can be occur either in isolation or in concert with one or more of the other issues, including a chamber mismatch.

I think goes back to how do you really know what caused the high pressure/popped primer/stuck case/what ever else the pressure sign was?

You can find out one of several ways. One could attempt an in depth analysis using the evidence provided by the incident and information regarding the possible causes.

Or one could eliminate possible causes one by one until one could not be eliminated. If the chamber is a .223 chamber and the ammo is 5.56 ammunition then one of the possible causes that should be considered is an ammo/chamber mismatch. Not because it's the only possible cause but because it is a known issue that can cause overpressure events.

Kinda like your stove idea . Just because the kid comes out of the kitchen with a burn on his arm does not mean it came from the stove . could have been the toaster or the lighter you some times use to light the stove . How about something that just came out of the oven and is no longer in or on the stove . Until you know all the data of what happened in the kitchen you really have no idea how they got burnt

Very well said. However, what we have on this thread, and what I have seen many times before is what amounts to people arguing that because there are many possible causes for burns and it's hard to prove what caused a particular burn, and because someone touched a stove once and it didn't burn them (stoves aren't always hot enough to burn) that we should automatically discount the idea that stoves can burn and we should, in fact, give little weight to warnings about the potential danger of touching stoves.

Lucky Gunner tested M855 ammo in two AR-15 rifles and a bolt action .223 rifle:

Right, and if you look at his pressure data in the article, some of it shows the mismatch resulting in pressures above SAAMI max. If you read to the end, he also recommends against the mismatch.

But that still doesn't give us enough pressure to cause a popped primer or locked up action, at least not on its own.

That's been covered more than just a few times on this thread. The pressure rise generated by the mismatch is often not sufficient to cause problems in the absence of other contributing factors. But it has been demonstrated to raise pressures above SAAMI max and there is strong evidence to indicate that it does play a part in some overpressure events. There is also evidence to indicate that in some cases it can cause overpressure events all by itself--as trigger643's experience shows. Reaming the chamber (eliminating the mismatch) eliminated the overpressure events even though nothing else was changed.

So you see with a delayed ignition problem, the longer freebore and shallower leade angle of a 5.56 chamber could avoid that pressure increase in the primary pressure spike. Alternately there is no chamber that can mitigate a secondary pressure spike, which seems to me to be the most likely suspect for 5.56 ammo issues in 5.56 chambers.

I'm going to take the high road and NOT ask you to quantify "most likely".

As I've said more than once, there are certainly other issues that can cause overpressure events. Depending on the particular circumstances of the event, one issue might be more probable than another.

As far as the secondary pressure spike issue, given that your source discounts the possibility of a significant secondary pressure spike in guns with barrels 20" or shorter, I'm going to have to disagree with you that it's the "most likely" cause of overpressure events when there's an ammo/chamber mismatch. In fact, I'm going to disagree with you that it's the "most likely" cause of overpressure events in any typical AR given that the barrel length in the typical AR is 20" or shorter.

 

[Primses]

A 5.56 can fire .223 no issue. The other way around can cause damage to the gun do to 5.56 having a much higher loading pressure. I have heard of receivers getting destroyed but people fire 5.56 our of mini-14's and those are .223 only. Still don't chance it.

The only real difference besides the lower velocity is that the round is not as tight for lack of a better explination so the accuracy will suffer. Not much, but the people who shoot at long distances and care about 1/8" at 200 yards would notice.

 

[Jimro]

JohnSKa,

I'm going to take the high road and NOT ask you to quantify "most likely".

You could ask all you want, I've already said that I don't have the data to quantify factors. I really wish I did.

But as you yourself noted, you can find out one of several ways. One could attempt an in depth analysis using the evidence provided by the incident and information regarding the possible causes. Which is why we've been talking pressure trace and case dimensions.

Reaming the chamber (eliminating the mismatch) eliminated the overpressure events even though nothing else was changed. The pressure rise generated by the mismatch is often not sufficient to cause problems in the absence of other contributing factors.

But what we don't know is whether that was that due to mitigating an ignition delay in bad ammo, bringing an out of spec chamber (improperly cut at the factory) into spec, or just allowing more gas blow by during the firing process? Without access to data about how the ammo performed in the original chamber we don't know.

However, the existence of other issues that can cause abnormally high pressures is not in debate. Everyone acknowledges that they exist and that they can be occur either in isolation or in concert with one or more of the other issues, including a chamber mismatch.

As far as secondary pressure spikes "should not happen" in 20" barrels or less, please remember that the LuckyGunner article shows those secondary pressure spikes happening with 5.56 ammo in 5.56 chambered ARs, and a secondary pressure spike can happen AFTER the bullet leaves the bore. In the face of this evidence, can you continue to dismiss a secondary pressure spike as a source of an overpressure event?

http://www.shootingsoftware.com/images/Nosler.jpg

Jimro

 

[Brian Pfleuger]

...due to 5.56 having a much higher loading pressure.

This is simply not true. The problems are with the slightly different chamber/throat dimensions and not with higher or lower design pressure limits. The pressure limits on the cartridges are almost entirely due to pressure gauge placement than any difference in real pressure. Even if the rated differences were real, 5-6,000 psi is not enough to make any safety difference at all.

They are rated:

CIP 5.56NATO 58,740 PSI

SAAMI 223 Rem 55,000 PSI

CIP .223 Rem 62,366 PSI

 

[Jimro]

To add to Brian's post,

CIP 5.56NATO 58,740 PSI in a 5.56 Chamber, measured at case mouth

SAAMI 223 Rem 55,000 PSI in a SAAMI 223 minimum Chamber, measured middle of the case

CIP .223 Rem 62,366 PSI in a CIP 223 Chamber, measured at case mouth. Patrick Sweeney claims that the CIP 223 Chamber is the same as the 5.56 Nato chamber but I have seen no evidence of this other than Sweeney's assertion.

Here is a good explanation of how the differences in measuring pressure can lead to some confustion: http://ballistictools.com/articles/5.56-vs-.223-myths-and-facts.php

Jimro

 

[JohnKSa]

...people fire 5.56 our of mini-14's and those are .223 only...

The Mini-14 rifles actually have a 5.56 chamber in spite of how they're marked externally.

But what we don't know is whether that was that due to mitigating an ignition delay in bad ammo, bringing an out of spec chamber (improperly cut at the factory) into spec, or just allowing more gas blow by during the firing process? Without access to data about how the ammo performed in the original chamber we don't know.

However tortuously one chooses to describe it, the fact remains that there WAS some sort of ammunition chamber mismatch that was causing overpressure events in the absence of other factors and that when the chamber was cut to the proper specs the overpressure events stopped.

One can create a Rube Goldberg explanation to try to deflect the investigation/conclusion away from the obvious, but it doesn't change the fact that the basis of it all is an ammunition chamber mismatch of one sort or another that is causing overpressure events.

Could it possibly have been something else? It's possible--anything is possible, after all. But is it likely? Only if you really, REALLY want it to be... Ok, no it's still not likely, not even if you really, REALLY want it to be. How bad someone wants something doesn't change reality.

As far as secondary pressure spikes "should not happen" in 20" barrels or less, please remember that the LuckyGunner article shows those secondary pressure spikes happening with 5.56 ammo in 5.56 chambered ARs, and a secondary pressure spike can happen AFTER the bullet leaves the bore. In the face of this evidence, can you continue to dismiss a secondary pressure spike as a source of an overpressure event?

If you'll note, the secondary pressure spikes in the shorter barrels tested by lucky gunner are, in ALL cases:

1. Lower than the primary pressure spike for any single discharge pressure curve.
2. Below SAAMI max.

The other article states that they should not be an issue in barrels 20" or shorter, and luckygunner's data strongly supports that. The graphs you keep showing that have off-the-charts secondary pressure spikes are not from 20" or shorter barrels.

So why do I think secondary pressure spikes are a less likely explanation? Because, according to both sources you provided they don't result in pressures above SAAMI max in barrels under 20", while the ammo/chamber mismatch clearly can.

 

[Jimro]

JohnSKa,

Allow me to put in the exact quote.

The above examples of severe secondary pressures should not occur in a 20 inch long tight bore.

That we saw secondary pressure spikes in the LuckyGunner article shows that they can happen in shorter barrels without a chamber mismatch. What we can't say is that had the powder been different, such as 2230 which is known to produce secondary pressure spikes that have been noted to be strong enough to expand bores.

However tortuously one chooses to describe it, the fact remains that there WAS some sort of ammunition chamber mismatch that was causing overpressure events in the absence of other factors and that when the chamber was cut to the proper specs the overpressure events stopped.

One can create a Rube Goldberg explanation to try to deflect the investigation/conclusion away from the obvious, but it doesn't change the fact that the basis of it all is an ammunition chamber mismatch of one sort or another that is causing overpressure events.

I actually think that this is good progress. You are acknowledging that the standard "sharper leade angles cause pressure spikes" explanation doesn't explain all the symptoms we've seen. However this still doesn't explain how popped primers have been known to happen with 5.56 chambers, and generally the accepted wisdom at that point is "bad lot of ammo." Or as Patrick Sweeney put it, "Max charge of powder heated beyond safe with a case head barely hard enough to avoid lot rejection."

I am inclined to the explanation that the more generous leade is covering up for ammunition defects such as delayed ignition, and the reason we don't see popped primers/pressure signs with every pull of the trigger is that not every round in a lot will necessarily have the same defect. I can't really prove that without more data.

So why do I think secondary pressure spikes are a less likely explanation? Because, according to both sources you provided they don't result in pressures above SAAMI max in barrels under 20", while the ammo/chamber mismatch clearly can.

And yet none of the pressure traces of the mismatch showed pressure high enough to pop a primer, which a secondary pressure spike could easily do if the bolt is in the extraction cycle even if it is below SAAMI max pressure. Without a case head measurement to determine if there was an overpressure event a popped primer we can't determine if it was due to the case head not being supported against the bolt face. Milspec ammo has crimped primers for a reason.

Jimro

 

[JohnKSa]

That we saw secondary pressure spikes in the LuckyGunner article shows that they can happen in shorter barrels without a chamber mismatch.

Let's look at what your source says again:

"The above examples of severe secondary pressures should not occur in a 20 inch long tight bore."​

That is ENTIRELY consistent with what the luckygunner pressure traces show. While they are clearly secondary pressure spikes, they are also just as clearly NOT "severe secondary pressures" given that they don't exceed SAAMI max.

Furthermore the FN barrels in the testing were 20". Right at the threshold for the spikes (according to your other source) while the most common AR barrel length is shorter than 20".

It's clearly stretching to try to make a phenomenon that doesn't raise pressures above SAAMI max even in 20" barrels and is even less likely to cause significant pressure rise in the more common shorter barrels the culprit.

That's especially true when we have another phenomenon known to raise pressures significantly above SAAMI max that fits the data much better.

And yet none of the pressure traces of the mismatch showed pressure high enough to pop a primer...
...
...a secondary pressure spike could easily do if the bolt is in the extraction cycle even if it is below SAAMI max pressure.

In point of fact, none of the secondary pressure spikes popped a primer during the testing in spite of your speculation that it could happen even with the pressures shown.

And, again (for about the fourth or fifth time), we DO know that the mismatch CAN pop primers although the data and information available does indicate that it's somewhat unlikely to do so in the absence of any other contributing factors.

You seem to think that if you keep saying that it can't pop primers that your steadfast repetition of that mantra will somehow make it true. It won't and it hasn't.

I actually think that this is good progress.

It doesn't surprise me that you would think that. You have repeatedly demonstrated that you see only what you want to see in what you read. If it supports what you want to be true, it apparently stands out, if it does not, you can read it repeatedly and apparently never even see it.

It's not progress at all in any sense, if by "progress" you mean that I'm coming around to your point of view. I've made it as plain as I can make it that I don't believe you are objectively evaluating the facts and information available and I haven't changed that opinion.

I can't really prove that without more data.

No, of course you can't. I'm going to go out on a limb (though not very far at all) and say that you couldn't prove it even with more data. The funny/sad thing is that with the data you already have you could go a LONG way toward DISproving it if you were even slightly inclined to view the data objectively.

 

[Jimro]

JohnSKa,

Please explain why the 7mm STW secondary pressure spike happened after the bullet left the bore. http://shootingsoftware.com/images/Nosler.jpg
Then you can compare it to this bolt action 223 which has the secondary pressure spike as the bullet exits the bore: https://www.shootingsoftware.com/images/sampletracebump.gif and then http://www.luckygunner.com/labs/wp-content/uploads/2012/06/silverbear-223.png which doesn't have a crossbar noting bullet exit, but should be between 1.0 and 1.5 milliseconds.

Now explain why a secondary pressure spike can't happen based on barrel length, especially since we know they can, in ARs. Even with those "20 inch long tight bore" FN barrels. Yes they weren't as severe as the other example noted, but they still happened while the bullet was in the bore, which in the last 5.5 inches of the bore corresponds to "out of the bore" for a 14.5" barrel. If you can get a secondary pressure spike after the bullet leaves the bore in a 7mm STW, then it stands to reason that you can get a secondary pressure spike after the bullet leaves the bore of a 5.56.

Remember that to pop a primer once the case head isn't locked against the bolt face you don't need above SAAMI spec pressures.

And all of this could be falsified if you could show a primary pressure curve that was above proof level loads due to the mismatch. But you can't, because all the data we have available doesn't show a dramatic increase in pressure in the primary pressure curve. I honestly expect you to sidestep the lack of available data and respond with "well the experts say it does, and we should trust the experts."

Once again, I'm not arguing anyone should mismatch their ammo. I am saying that the common wisdom explanation is incomplete.

I have noted that the "mismatch syndrome" shows up more often with M193 loads instead of M855 or Mk262 heavy loads. This is in line with ammunition pushing a light bullet with too slow a powder. This is mainly based on me scouring the AR15.com archives to look for reports of "chamber mismatch" and either the denizens of arf.com have a conspiracy to use only the cheapest M193 ammo they can get their hands on, or the problem isn't showing up as often with other loads (save as noted the known issues with a bad lot of Radway Green SS109 even in 5.56 chambers).

Jimro

 

[JohnKSa]

Now explain why a secondary pressure spike can't happen based on barrel length...

Perhaps that's a request you should address to the source you chose to quote as an authority on the topic.

This is a classic example of the way you have selected data you like and discarded data you don't like, even from a single source.

For what it's worth, I did notice that you tried to sneak a strawman into the question. Your source does not say they "can't happen based on barrel length" it says that SEVERE secondary pressure spikes won't occur in barrels 20" or shorter.

Even with those "20 inch long tight bore" FN barrels.

The data agrees with the statement. The secondary pressure spikes in those barrels were well under SAAMI max and lower than the primary pressure spikes.

What it boils down to is that the whole idea of the secondary pressure spike phenomenon being a more likely explanation for the issues which three ammunition companies, et. al. attribute to the ammo/chamber mismatch is nothing more than speculation on your part.

What's more cogent is that it's not even really a good explanation for the issues unless one has chosen, as you have, to reject the ammo/chamber mismatch explanation in spite of the evidence to support it.

1. The very sources you're basing your speculation on indicate that there shouldn't be severe pressure spikes in barrels 20" or shorter and confirm that with pressure data showing that the secondary pressure spikes in 20" barrels are below SAAMI max.

2. The whole secondary pressure spike issue is theorized to result from mismatching too-slow powders with very light bullets. Something that might happen in handloading (coincidentally the source most of the data showing severe secondary spikes) but that is less likely to occur with factory/military ammunition.

3. If we want to attribute the ammo/chamber mismatch issues to secondary pressure spikes, we would need to come up with some reasonable explanation for why the 5.56 chamber would eliminate the secondary pressure spikes.

Is it one POSSIBLE cause for some overpressure incidents? Sure, it's one POSSIBLE cause. Is it a more likely explanation for popped primers and other over pressure events in cases where there's a known ammo/chamber mismatch. Not by any objective reasoning process. At least not based on the evidence/information/data available at this time.

 

[Jimro]

JohnSKa,

Do you have any pressure traces showing a primary pressure spike above 70k due to a chamber mismatch?

Jimro

 

[Metal god]

...due to 5.56 having a much higher loading pressure.

This is simply not true. The problems are with the slightly different chamber/throat dimensions and not with higher or lower design pressure limits. The pressure limits on the cartridges are almost entirely due to pressure gauge placement than any difference in real pressure. Even if the rated differences were real, 5-6,000 psi is not enough to make any safety difference at all.

This seems to be talking in circles then . If the 2 cartridges are loaded to the same pressures and if different not enough to be dangerous . Then what is all this back and forth all about . I asked the question a few pages ago about load data in my reloading books .

Pull out any of your manuals that have both 223 Rem bolt data and AR 5.56 data . Now compare apples to apple . Meaning check out the loads that use the same bullets and powders . What do you see as far as max loads for each of those chambering .

What I see in both my Hornady and Sierra books is the 223 is loaded to at least the same max powder charges and many times the 223 will allow a heavier charge then the 5.56 .

So if the 5.56 is loaded to the same pressures or just over as the 223 Rem why is there an issue for mismatched chambers ??? If the only REAL difference in the cartridge is the head stamp what's the problem ???

Are 5.56 rounds loaded with more powder and or have higher pressures if measured in the same way ?? This seems to be all that matters here . I think we all agree the the closer you seat the bullet to the lands the higher the pressure spike can be . How ever if both cartridges are in fact loaded the same and the only difference is how the pressure is measured . There should be no mismatch issues .

Is the 5.56 round loaded hotter then 223 Rem ? My books say no if your reloading . Even if the books are on the low side to be safe . I would think they are low with both rounds making it a wash as far as this debate goes .

IMHO the real issue is ,

Is the 5.56 NATO round loaded with more powder ?
Does the 5.56 NATO round have a hotter faster burning primer ?
If measured in the same way how different would the pressures be ?

If the 5.56 is not loaded DIFFERENT then the 223 Rem what's the problem ?

 

[Brian Pfleuger]

This seems to be talking in circles then . If the 2 cartridges are loaded to the same pressures and if different not enough to be dangerous . Then what is all this back and forth all about . I asked the question a few pages ago about load data in my reloading books .

The design pressures are not different enough to cause problems. The chambers are different enough to cause pressures beyond design pressures.

A suitable, though not perfect, analogy would be like 40S&W and 10mm. 40SW rounds would fire perfectly well in most 10mm guns. The extractor would hold the round tight enough to ignite the primer, fact is that's how most semi-auto handgun rounds "headspace" anyway. Do it the other way around though, if you could get a 10mm round to chamber in a 40SW gun and you could have major problems.

It's not the design pressures that are so radically different, it's the CHAMBER DIFFERENCES that cause the problem.

As you can see from the explanations above regarding pressure ratings and gauge placements, the actual design ratings are, for all intents and purposes, identical.

There are many similar examples. .308Win and 7mm-08 Rem are essentially identical cases and rated for essentially identical pressures. It's not the pressure rating that's an issue, it's trying to cram a .308 bullet down a 7mm bore.

Specific to .223/5.56, the chamber difference can raise pressures by having (among other things) the bullet from a 5.56 round jammed into the rifling of a .223 chamber, not because the design pressures are a problem.