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5.56 and .223: Are They Different?

by Patrick Sweeney   |  February 2nd, 2012 21
shooting an m4

We think of them as the same, but the 5.56 NATO and .223 Remington are different enough to matter.

You don’t have to play in the AR playground very long before you hear or ask the question; “Does .223 vs. 5.56 matter?” The argument can reach near-religious proportions, with some on both sides vehemently arguing their cases. The deal is this: They do differ, and, yes, it does matter.

The differences really come into play with the starting path of the bullet and the unrifled portion of the chamber ahead of the rifling, called the leade. A narrower (less diameter) leade keeps the bullet from tipping as it moves forward, which enhances accuracy. A larger leade allows for more buildup and gunk and thus greater reliability.

At the end of the leade is the rifling. To prevent bullet mangling, the rifling has an angle on its front face to allow the bullet to slide in and smoothly begin rotating. The steeper this angle (within reason) the more quickly the bullet is controlled by the rifling and thus potentially delivering greater accuracy. However, the steeper the angle, the more the bullet “stalls” on engaging the rifling and the greater the pressure spike.

5.56 chamber

The 5.56 chamber has a slightly longer leade and shallower rifling angle, allowing the 5.56 cartridge to be loaded to higher pressures.

A shallower angle on the rifling—in effect, a longer ramp—also creates a longer effective leade, as the ogive of the bullet has to travel a greater distance before it engages shallow-angle rifling than it would with steep-angle rifling.

When the .223 was invented, it was meant as a benchrest and varmint round. As such, accuracy was paramount. Velocity was a good thing, but not if it meant losing accuracy. So the .223 was designed with a short leade and steep rifling engagement, which is fine for shooting varmints or targets.

When the Army was forced to adopt the M16, however, it tried to avoid adoption by moving the goalposts, insisting on 500 yard penetration performance. To get that, the designers had to boost pressure and velocity. To control pressure (and also to get full utility out of tracer ammo, which uses bullets nearly twice as long as typical full metal jacket ammo), they modified the shape of the leade and rifling angle. And later, taking advantage of the longer leade and gentler angle, ammo makers tuned the 5.56 round to maximum performance using that extra margin.

.223 Remington chamber

By contrast, the .223 Remington has a shorter leade and sharper rifling angle, the design stemming from a desire for top accuracy.

Today, the difference can be marked. The leade on a proper 5.56 chamber is twice or more than that of a .223 chamber, and the onset angle of the 5.56 rifling creates a ramp with four times the distance. Firing .223 in a .223 chamber, or a 5.56 chamber, is not a problem. But firing real-deal 5.56 ammo in a .223 chamber can be a big problem.

The SAAMI-spec maximum average pressure for the .223, measured at the middle of the case, is 55,000 psi. The NATO spec for 5.56 is determined by SAAMI’s European counterpart, CIP. CIP measures at the case mouth and lists the 5.56 pressure spec of 62,000 psi. Measured at the case middle as SAAMI does, it shows 60,000 psi—so either way it’s higher than the .223.

But the problem isn’t just pressure. That CIP pressure of 62,000 psi? It is measured in a 5.56 chamber. If we take the same round, which shows 60,000 psi per SAAMI (which is already 5,000 psi over the .223 max) and put it into a .223 chamber, things get ugly. The pressure spike piles onto an already over-pressure round.

  • Thomas Deal

    What about this wylde chmber that some of the manufactures are saying will wok for both 223 and 556 ?

  • jack deatherage

    yeah, what about the wilde chamber? thanks.—

  • Big Bird

    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.

  • Thomas Deal

    I bought a rock river predater pursuit upper with a wylde chamber and just wanted to know what it was all about. It shoots really well with 50gr v-max at 3200fps .I would like to get a little more out of it but was worried about the pressure thing !

  • tmnolan

    Does it make a difference if I shoot 5.56 ammo in a bolt action rifle marked .223 Remington?

    • Wise One

      Read the F**King article

  • JRLutz

    Bolt Action you will be fine as all 223's are tested to a min of 75,000 psi , and a 5.56 is around 62 give or take a thousand. The Wise One should have been more diplomatic, I am sure his shooting is better than his manners are.

    • JBenat

      Lets hope so! People don't read these articles to get their ears pinned back by some faux expert.

      • MAX

        I agree, but I can still see where he is coming from. Leade has nothing to do with the action. The way he went about saying that was then irritating part. Reader, please remember that no matter you expertise (or lack thereof) there will always be someone who knows less than you about this. Learn to tolerate this, and to bring them along the path of knowledge in a patient manner. Scorn doesn't help anyone.

  • http://www.facebook.com/profile.php?id=100000500081415 Tim Cas

    Utter pish

  • J Gleason

    Although the detail of the article is a little more technical than most shooters-gun owners are able to digest,it was a good article. Thanks!

  • Tim

    I don't see the problem of having an AR chambered in 223 Rem(as per the 2nd to last paragraph). Most of the shooters that I know (including myself) don't care if their rifle is "mil-spec". What they do care about is accuracy and reliability, which they find with the 223 chambering. If you ask me, it sounds like Mr. Sweeney has a little bit of a chip on his shoulder.

  • kevin flesner

    Timely article for me. Just purchased Rem 788 at auction. Bought several different loads for it to test and sight in. One of which was bargain brand steel case 62 gr HP marked as .223, not 5.56. 3 rd round stuck un chamber. Cleaned chamber in case I didn't do good job on initial cleaning of rifle. Next round also stuck. Upon beating bolt open, found flattened primer as well. Believe these are 5.56 rounds in spite of markings on box.
    Gave the rest to a relative with an AR!
    Rest of shoot went well with brass casings and 50 – 55 gr loads.

  • Texas_shooter

    To get the best accuracy from my .22-250 handloads, I would seat a bullet long with no crimp. Then chamber it in my target rifle and set this as my overall length for this rifle. The bullet would just touch the rifling. If I changed brands of brass, I would re do this for safety. Stoner never intended the M16 to be 500 Meter weapon, he did intend for it to be very accurate at shorter ranges. The military did much, much more to discredit this fine weapon. So, I can see this happening. Stoner's M16 was chambered for .223.

  • Jerry

    Have same type of problem with 6.8 spc ammo with the various 6.8 chamber sizes in various rifles.

  • Jarhead

    Has anyone throat-gauge tested a Rock River AR-15 barrel? I have one (marked 5.56 mm) and am wondering if it is chambered as .223 or truly mil-spec 5.56 mm. I certainly can see advantages to either and would not be upset to find that it is one or the other, but would just be nice to know.

    Great article, thanks!!

    • MAX

      If it from RRA, the markings are probably spot on. Shoot whatever in it, but the 5.56 should perform better.

  • Kirk

    I bought the reamer and gauge from M-guns. Top quality tools. Three of the four AR rifles I own and all three bolt action rifles had .223 chambers. About 50% of the AR rifles I have checked have had ,223 chambers. I shoot a mixture of .223 and military spec. 5.56 in my rifles and in my Contender. I do not believe that it is wise to shoot Proof loads in my weapons. The new 70 grain 5.56 ammo sets the bullet out even further into the .223 leade. The possible spike to 75,000 cup will cause the shells to stick in the chambers causing extraction problems at the best and damage to the rifle and injury to the shooter at the worst. The chamber gauge iis $40 and the reamer is $240. You can't buy a rifle or an eye any cheaper, I'm thinkin', but what do I know?

  • Edward Smith

    If you want a .223 caliber rifle buy one and use .223 ammo. If you want a 5.56 rifle buy it and use either one.

  • rick

    I have a bushmaster marked 223 – 5.56 . What round would be best and safest to shoot?

  • scott

    My new ruger ranch mini14 in the mail as I type. All but the target rifle is supposed to be 5.56 chambered. I noticed Ruger was not in the names listed in the article. Does anyone know if these are true 5.56, saving me the time of guageing it when it arrives.

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