5.56mm NATO versus 223 Remington:  Chamber Differences


A SAAMI spec .223 Remington chamber will have a shorter leade with a sharper angle to the leade and a shorter amount of effective freebore than a 5.56mm NATO chamber. The freebore itself will also be narrower in the .223 Remington chamber.


raw pics courtesy of Ned Christiansen


With all other things being equal, the 5.56mm NATO chamber with its longer and shallower angled leade and longer amount of effective freebore will produce less chamber pressure than a .223 Remington chamber, when firing .223 Remington SAAMI Spec ammunition. Because of this, 5.56mm NATO amunition can be loaded to a pressure that would be considered excessive in a .223 Remington chamber, yet acceptable in a 5.56mm NATO chamber. (Pressures are measured using different methods between the two systems.)


The leade of a SAAMI 223 Remington chamber.






The official SAAMI 223 Remington drawing.








From Jeff Hoffman, co-owner of Black Hills Ammunition

“The 5.56 IS a higher pressure/velocity cartridge, but it is made to a military standard, with different test methods, (and therefore is not easily directly comparable to .223 pressures) . . . the general spec for US 5.56 ammo is 58,700 psi max, measured at case mouth. . . Please note this is a different method than SAAMI transducer or copper crusher, as used on commercial ammunition. 5.56 ammunition spec results in ammunition loaded to a higher pressure level than commercial .223, but the test methods specified are different . . .The spec calls for a different pressure test method than SAAMI spec ammo, and is not directly comparable . . . “


Here's a very interesting quote posted by Ned Christiansen on M4carbine.net:


". . . In short, you can safely fire all 5.56 AND 223 ammunition in a gun properly chambered for 5.56. You MUST NOT fire 5.56 ammunition in a 223 rifle. As case in point, I fired XM193 5.56 ammunition in a 223 test barrel with average pressures (conformal transducer) of 72,550 psi, and peak pressure registered at 76,250 psi. . ."

Since the SAAMI MAP for the .223 Remington is 55,000 PSI, that puts XM193 fired from a minimum spec .223 Remington chamber at 17,550 PSI over the maximum.


(The following is just a generalization to give a graphical demonstration of concept. DO NOT hold me to the exact numbers as they are not correct and they ignore the difference due to the different methods used to measure chamber pressure.)

Consider the left graph pictured below; M193 fired in a 5.56mm chamber. The pressure is within the MAP limit. Now, take the exact same round, (same powder, same charge of powder) and fire it from a .223 Remington chamber; pictured in the right graph below.

Because the .223 Remington chamber has a shorter and sharper angled leade compared to the 5.56mm chamber as well as a shorter effective free-bore, the bullet engages the rifling sooner in the .223 chamber than it would have in a 5.56mm chamber. This causes the pressure to rise faster, peak sooner and reach a higher (and per SAAMI, unsafe) level than it would have if the round had been fired from a 5.56mm chamber.





Many people seem to incorrectly believe that 5.56mm brass cases have significantly reduced case capacity compared to .223 Remington cases. The table of case capacities shown below demonstrates that this notion is false.












.....

To add to Molons post, many manufactures, i.e. Rock River Arms, Larue Tactical and others run a Wylde chamber.

What's a Wylde chamber?
The .223 Wylde chamber was designed as a match chambering for semi-automatic rifles. It will accomodate both .223 Rem and 5.56mm NATO ammunition. It is relieved in the case body to aid in extraction and features a shorter throat for improved accuracy.

What specifically is "relieved" in the case body. How much relieved over regular dimensions?

You have probably heard this a million or two times but I am new to the AR style rifles and I have two one for my grandson and one for me, an AP-4 and a Mini Sass, I will not mention the manufacturer as maybe that is a no no here, I am also new to AR15.com. I was interested in the tested pressures of these rifles and also just what type of chamber were cut in them, they are marked on the barrel 5.56 however I understand that there are different styles of chambers cut into these rifles and could be any of them. I then called the manufacture and ask a tech those two questions, the answer I got back just left me with a question mark, his responce was they are both 5.56 and the pressure you can get from the SAAMI web site??? What???  he would not go any further with me and I was nicely dismissed as he had no further comment, is this normal? what the hey!!  Maybe these are stupid questions I dont know and what does the SAAMI web site have to do with NATO chambers? I have been trying to read as much as possible about the AR, must not have learned enough. Any insight I can get on this would help because I would like to reload for these rifles.

Thanks for posting this. It clears up the question of .223/5.56 for me in a very straightforward way.

Stupid browser.

With this info in mind and noticing how HOT barrels get with just 50 rounds, IMO, running 5.56x45 in a .223 designed barrel is going to result in some fatigue further down the line.
Unlike my pistols, it appears AR-15 style rifles need closer attention when either purchasing or reloading ammunition. Thanks for this invaluable information.

On a somewhat related note, I've noticed a few head-space checkers on the internet (MidwayUSA or Brownells) and wondering if it is worth getting the GO and NO-GO checkers? My first carbine just arrived yesterday (no, I didn't assemble mine) but I will be doing my own assembled AR-15 later in the year so as part of the "tool kit" so to speak, was curious about these checkers.

Suggest sticky and FAQ.  Great info as usual!

This is a great explanation and the photos really show the difference.  Nice work.  But the one question that I can't seem to find an answer for that I really  trust is about my early (1970's) Colt SP1.  It's marked 223/5.56.  I called Colt to ask about the NATO ammo and got an "it's all OK" kind of answer.  Didn't the NATO chambering start AFTER my SP1 was built?  Or more specifically, how were the Vietnam era barrels, such as my old SP1, chambered?  Should I stick to XM193 ammo or just shoot anything it?

Thanks, jim.

So I get it's "bad" to fire a .223 round in a 5.56 chamber, but ok going the other way.

I guess I'm a bit confused because of the markings on my rifle.....My Bushmaster AR-15 is marked .223/5.56 on the lower and the barrel is marked MP 5.56 with 1/9 twist. My question is....what markings should I really be going by to be on the safe when buying ammo? It's not modified in anyway, Bought it new in factory sealed box from a friend/local dealer. My friend has been selling me .223 and 5.56 ammo for it. I have shot both thru it with no issues so far.

Your Bushmaster is chambered for 5.56 so you can safely shoot both that and .223.

Thanks for clarifying that, much appericiated!!....I either read the OP wrong at that time or things got switched around in my brain while I was typing that...lol

The markings on the lower are immaterial; many lowers are marked "MULTI" because there is little beside magazine well dimensions that has anything to do with what cartridge a rifle is chambered for.  On the other hand, the barrel's markings say without question, not just what cartridge the chamber is cut for, but (usually) the rifling twist and sometimes even the kinds of testing (MP, HP, etc) the barrel has undergone.

This seems contrary to everything I've read abou 5.56 vs .223.  Thoughts?

http://www.armalite.com/images/Tech%20Notes/TECH%20NOTE%2074%20%205.56%20vs%20223%20vs%20Wylde%20090817%20Rev%200.pdf

Nice to see you post in the tech forum again. Your regular informative posts are sorely missed.

It seems 'they' are taking over and it's a damn shame.  
 

Great post. This was helpful. Thanks Molon.

The tech was an idiot.  The 5.56mm cartridge was never accepted by SAAMI, so there is no SAAMI MAP for the 5.56mm cartridge.  


...

I would seriously like to see someone fire M193 and M855 in a 5.56mm test barrel using SAAMI pressure testing methods.  This would allow us to compare apples to apples, so to speak.  And determine whether or not I could rechamber one of my 223 Contender barrels to 5.56mm.

I hope that poor fella hasnt been waiting by his computer for the last 4 years waiting on an answer... On the other hand if he has Im sure he's happy he got an answer!!!
Lot's of good info here....

So when is Molon getting his own forum of all his threads as a reference forum?

 
Ditto this. I'm running out of room tagging these threads. Great information and insight. Helps dispel a bunch of those gun store rumors.

This isn't exactly what you asked for, but someone has tested different ammo and chambers using the same pressure measurement.
http://www.luckygunner.com/labs/5-56-vs-223/
Nutshell:
pressure is higher in milspec ammo, regardless of chamber. It's .223 +P+
"5.56" in a "5.56" chamber is still higher pressure than "223" in a "223 chamber"
the style of chamber may or may not make a small difference in pressure. individual variation between chambers may make as much difference.

This last point needs to be stressed.  If do some digging, you will find that sweeping statements about what is safe to use in what bear at times only a vague resemblance to the truth.

Just because a barrel is marked "5.56" or ".223" does NOT mean that the chambers were cut to Molon's specs as noted.  Just becuaee a barrel is marked .223 doesn't mean it IS.  The only way to really know, is to do a chamber cast.  That is a bit of an overstatement in that there are makers who adhere closely to SAAMI spec for example, but anybody who has rechambered rifles to various calibers knows there is a fairly wide variance in ACTUAL resulting chambers with a reamer marked to a specific caliber.

I suspect this is why lots and LOTS of ".223" marked rifles are shot with LOTS of 5.56 ammo and we hear of very, very few kabooms, not to mention the super-adequate strength of the rifles involved.

If somebody is wondering if chamber dimension are all over the map, well, yes, they are.  No, I'm not talking about a .223 being more like a .22-250 than a 5.56.  I am talkingabout specs that push the tolerance limit or exceed it for SAAMI .223 chambers.  As for "5.56" chambers, I'd LOVE to see chamber casts of numerous rifles marked as such, both military and civilian.  I think it would widen some eyes.

Until the recent internet-based obsession with the topic, manufacturers generally considered the terms interchangeable and not indicative of the minutia of how the chamber throat was cut.. Some manufacturers intentionally marked things with 223 for reasons like some countries ban firearms in "military" calibers but "civilian" calibers are ok. Years ago it was believed (with some justification) that US sporting market consumers (aka "fudds") would hesitate to buy firearms with metric caliber designations.

Very few if any semi auto rifles are or ever have been made with a true SAAMI 223 chamber cut. The SAAMI 223 chamber has an interference fit at the throat, and the throat is shorter than a Wylde so it's very possible with some bullet shapes to have live rounds stick in the throat. With a bolt action it would generally not be a problem to eject a live round if you'd even notice it, but with the mechanical disadvantage of the initial unlocking and extraction on most semi autos that might be problematic. So for pure user-cycle functionality reasons a true SAAMI chamber would be avoided by most manufacturers. My strong suspicion is that is the reason for the answer given to the Mini-14 question above for example.

Yup.

Who knows what the exact chambering of the mini is, but I believe they have always approved that any .223/5.56 ammo can be used.  Similarly to the "commercial" vs "military" chamberings of 7.62x39 that Bill Ruger sloughed off, saying the Mini-30 could handle any of them.

I assume that he knew what he was talking about, which is to say I assume the chambers of both of those rifles are close enough to "5.56" to be unworthy of concern.

That actually makes a lot of sense and suddenly explains a few things in my little collection.
I know my Tavor is marked ".223 Remington" on the side of the body but the barrel clearly states "5.56 NATO" and I had wondered why they'd do that difference. Considering it is also sold to places with stricter guns laws and banns on ammo, and I know IMI/IWI has dealt with similar things in Europe (such as the 9x21mm cartridge and chamberings), the concept does make sense.
It would be neat to know if this was also a factor in why the Colt SP1's were marked "CAL .223"

Thanks for posting that link.  The chart labeled "Piezo Transducer Max Recorded Pressure" a little more than half way down the page actually answered my question.

Just to clarify all of this information concerning .223 ammo/weapons versus 5.56 ammo/weapons. I am considering purchasing a 1977 SP1 that is marked .223. From what I have read here, it is not advised to fire 5.56 ammo in this weapon. Is this correct?

That depends on exactly where the ".223" marking you're talking about is.  The lower's caliber marking has little if anything to do with the actual chamber.  See what the barrel is marked.

AFAIK Colt SP1 barrels were made exactly the same as their military barrels. You should be GTG.

As I mentioned earlier in this thread, in 1977 5.56 was just the icky metric system, and consumers would expect an Inch system measurement caliber designation. Without the internet nobody got obsessed with the minutia of chamber design or whether something was designated in Inch or Metric units.

As the test I also linked to indicates, the decades of hyperbole about "ZOMG 5.56" is vastly overblown.

There's no caliber or twist data on the "visible" areas of the barrel. Could it be under the hand guards? Any tips on getting a very stiff slip ring down? This is an unfired rifle and has most likely never been removed. Just watched a youtube demo using a rubber coated channel locks. I'll give it a try. And, there's an actual hand guard removal tool designed for it. Never knew that.

Somebody gave you some inaccurate information.  Here is what the man himself had to say.


"Posted By: Bill Wylde
Date: Thursday, 13 November 2003, at 8:46 a.m.
In Response To: Re: .223 Wylde

The case dimensions of the Wylde reamer are that of one of the NATO prints. Not a thing tight about it. As I recall, there were two NATO prints in use. I don't recall the print number used, but do have it in old records somewhere.

Throating was about the only change. The decision was made to make the freebore diameter .2240" as a good bullet seal. That done, excess freebore made little difference to accuracy. One of the reasons the magazine length 69's shot so well in the chamber. It so happened that the 80 grain Sierra seated to the lands was about ideal at .2470" OAL. Simple luck.....All of it.

The initial reamer(designed in 1984)was mainly geared toward Canadian 5.56 ball, as I was experimenting with their issue ball for competitive purposes. This operation was slow in getting off the ground, and really didn't start happening until about 1990. The use of 5.56 NATO ball in Canada was a short lived affair. Handloads were allowed in about 1994.

Just prior to this time the AR's were gaining great strength in the U.S. The military finally got involved. The rest is history.

You might ask how the the 62 grain 5.56 ball worked for Canadian LR prone shooting to 1,000 yds? It was supersonic in barrels of 28", and longer. It also was very competitive with the 147 grain 7.62 ball in use there at the time. The wind drift differential at long range (7.62/5.56) was about 15% in favor of the 7.62.

Those days were very interesting."