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Difference between 223 cal. and 5.56mm chambers

Discussion in 'Band of Glockers' started by isuzu, Jul 21, 2005.

  1. isuzu


    Jul 3, 2005
    North America
    To fellow BOGs:

    If you plan to change the barrel of your AR/M16, please be aware that .223 cal and 5.56mm chambers are very different from the two. I found this out when reading the FAQ at Bushmaster( I recall a friend of mine in the Philippines who was very excited to show me his M16 that he changed with a stainless target barrel. Little did he know that the barrel was chambered for the 223 caliber.

    5.56 cal chambers have a longer leade compared to the 223 cal. chambers. As we know the Philippines, 5.56mm rounds are very common, but not 223 cal. The difference may not be detected visually, but long term use of 5.56mm bullets in .223 cal chambers might create a KB due to increase in pressure caused by copper build-up in the chamber. It may not happen to a new barrel, but when the chamber gets a fair amount of copper build up, then things can go wrong.

    The FAQ at Bushmaster also states that you could use 223 cal. bullets in 5.56mm chambers but not the opposite.

    Just a heads up.:)
  2. Mang Danny

    Mang Danny

    Jan 11, 2005
    Be careful especially if you shoot full auto.

  3. isuzu


    Jul 3, 2005
    North America
  4. zorkd


    Sep 2, 2004
    first written and published by yours truly on .223 Thu Apr 11 2002

    We all know that the military uses firearms, how else do you think they work? And these firearms use ammunition, aka cartridges and less properly called, bullets. Cartridges such as the 7.62x51 and 5.56x45, both currently standard NATO rifle rounds were, believe it or not, not originally created to be used for military purposes. They were respectively known as .308 Winchester and .223 Remington. It was only through the adoption of NATO did they become elements of the war machine.

    Now here is the point of this wu, is there a vital difference between the .223 Remington, which for all practical purposes is a "civilian" round, and the 5.56x45 which is almost the exact same round but being labelled so is considered "military"? Also note that the specs for .223 Remington is established and published by SAAMI while it is NATO that does the same for the 5.56x45

    Here is the explanation.

    It is often asked whether a certain rifle features NATO (North Atlantic Treaty Organization) or SAAMI (Small Arms and Ammunition Manufacturers Institute) standard chambers, and whether it makes any difference.

    FACTS: .223 Remington (SAAMI standard) and 5.56mm (NATO standard) rifle chambers are almost identical. The difference is largely limited to the "freebore," the cylindrical space in front of the case mouth, and the "lead" or "leade," the tapered region that eases the bullet into full engagement with the rifling. NATO and SAAMI cartridges can normally be used interchangeably with no problem.

    The SAAMI chamber features a less freebore and a tighter leade, which normally provide better bullet fit and match-grade accuracy than the NATO chamber. It is wonderfully suited to match bullets. ArmaLite, for example, uses a SAAMI chamber in all ArmaLite and Eagle Arms match and hunting rifles, and most of its M15A2 and M15A4 standard grade rifles.

    Millions of rounds of NATO ammunition have been fired safely in Eagle Arms' and ArmaLite's SAAMI chambers over the past 15 years.

    Occasionally a non-standard round (of generally imported) ammunition will fit too tightly in the leade, and resistance to early bullet movement can cause elevated chamber pressures. These pressures are revealed by overly flattened or powder stains that reveal gasses leaking around the primer.

    The first few rounds of ALL ammunition, from whatever source or lot, should be checked for pressure and other signs of defect before firing large quantities. If you have a problem, you can generally bet that the ammunition meets neither SAAMI or NATO specifications.

    All ArmaLite 5.56mm barrels bear markings on the top of the barrel in front of the sight base. If no marking is present the barrel is a .223 Remington SAMMI standard barrel. ArmaLite has adopted a practice of using SAAMI chambers in its stainless steel match barrels and the NATO chamber in moly (phosphated) and chrome-lined barrels.

    ArmaLite's larger AR-10 rifles are all chambered with 7.62mm NATO chambers, so no marking is used. .308 Winchester (SAAMI standard) ammunition functions perfectly in the 7.62mm chambers.

    Armalite is only one of the brands of gun makers that make the AR-15 rifle. Others like Bushmaster, Colt, and Elisco make similar rifles and what is written here also applies.
  5. isuzu


    Jul 3, 2005
    North America
    got this from

    Q. What is the difference between 5.56×45mm and .223 Remington ammo?
    In the 1950's, the US military adopted the metric system of measurement and uses metric measurements to describe ammo. However, the US commercial ammo market typically used the English "caliber" measurements when describing ammo. "Caliber" is a shorthand way of saying "hundredths (or thousandths) of an inch." For example, a fifty caliber projectile is approximately fifty one-hundredths (.50) of an inch and a 357 caliber projectile is approximately three-hundred and fifty-seven thousandths (.357) of an inch. Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

    The chambers for .223 and 5.56 weapons are not the same either. Though the AR15 design provides an extremely strong action, high pressure signs on the brass and primers, extraction failures and cycling problems may be seen when firing hot 5.56 ammo in .223-chambered rifles. Military M16s and AR15s from Colt, Bushmaster, FN, DPMS, and some others, have the M16-spec chamber and should have no trouble firing hot 5.56 ammunition.

    Military M16s have slightly more headspace and have a longer throat area, compared to the SAAMI .223 chamber spec, which was originally designed for bolt-action rifles. Commercial SAAMI-specification .223 chambers have a much shorter throat or leade and less freebore than the military chamber. Shooting 5.56 Mil-Spec ammo in a SAAMI-specification chamber can increase pressure dramatically, up to an additional 15,000 psi or more.

    The military chamber is often referred to as a "5.56 NATO" chamber, as that is what is usually stamped on military barrels. Some commercial AR manufacturers use the tighter ".223" (i.e., SAAMI-spec and often labeled ".223" or ".223 Remington") chamber, which provides for increased accuracy but, in self-loading rifles, less cycling reliability, especially with hot-loaded military ammo. A few AR manufacturers use an in-between chamber spec, such as the Wylde chamber. Many mis-mark their barrels too, which further complicates things. You can generally tell what sort of chamber you are dealing with by the markings, if any, on the barrel, but always check with the manufacturer to be sure.

    Typical Colt Mil-Spec-type markings: C MP 5.56 NATO 1/7

    Typical Bushmaster markings: B MP 5.56 NATO 1/9 HBAR

    DPMS marks their barrels ".223", though they actually have 5.56 chambers.

    Olympic Arms marks their barrels with "556", with some additionally marked "SS" or "SUM." This marking is used on all barrels, even older barrels that used .223 chambers and current target models that also use .223 chambers. Non-target barrels made since 2001 should have 5.56 chambers.

    Armalite typically doesn't mark their barrels. A2 and A4 models had .223 chambers until mid-2001, and have used 5.56 chambers since. The (t) models use .223 match chambers.

    Rock River Arms uses the Wylde chamber specs on most rifles, and does not mark their barrels.

    Most other AR manufacturers' barrels are unmarked, and chamber dimensions are unknown.

    Opinion: In general it is a bad idea to attempt to fire 5.56 rounds (e.g., M193, M855) in .223 chambers, particularly with older rifles.

    Fact: SAAMI specifically warns against the use of 5.56mm ammo in .223 chambers. The .223 SAAMI specification was originally made with bolt rifles in mind.

    For more see the SAAMI website ammo warning.

    5.56 v. .223 Remington specification.

    Fact: The different manufacturer's chamber types are listed at length and in great detail at: The Maryland AR15 Shooters Site.

    Q. Which should I be looking for in an AR15, a 5.56 NATO or .223 Remington chamber?
    This is really a matter of the role for which you plan to use your AR. .223 Remington chambers will give you slightly better accuracy, which is important for a match or varmint rifle. Any loss of feeding and cycling reliability and the restriction against shooting military ammo isn't as important as the accuracy gains for a rifle used in these roles, because for these rifles, accuracy is everything. People who just want to plink or who plan to shoot military ammo (such as most of the cheap surplus ammo available), and especially those who may use their AR as a weapon, should choose 5.56 chambers.

    Opinion: Unless you have a reason to seek out .223 Remington SAAMI spec chambers, 5.56 NATO is probably the best solution. 5.56 NATO chambers still can have outstanding accuracy and give you more flexibility in ammo selection.
    Q. What is the circle-cross stamp on some of my ammo?

    The circle-cross Å is the NATO symbol. It indicates that the ammo was loaded in a NATO-approved facility and meets the NATO specifications for that round. Note that NATO specifications are not the same as US military specifications and that many NATO-approved rounds do not meet US military specs. US military specs (such as M193 and M855) have additional requirements, such as minimum velocities, that the NATO specs (like SS-109) don't have.
    Fact: There are some exceptions to this rule. For example, recent Lake City and Winchester M193 is loaded in cases marked with the NATO circle-cross. This is done simply to save money by having one production run of cases instead of two. M193 was never adopted by NATO; by the time NATO decided to standardize on 5.56mm, the SS-109/M855 ammo was available, and was adopted as the standard. M193 is still "Mil-Spec," it just isn't "NATO" spec.