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97powerstroke 09-09-2007 06:19 PM

Ok, stupid question, but I want to know
Just what is the difference between .223 and 5.56. I'm new to AR's and I thought they were the same rounds but that doesn't seem to be the case. Sorry for the dumb question but I would rather know then not know.

bkt 09-09-2007 10:19 PM


Almost a quarter of a century ago, SAAMI recognized potential problems with shooters assuming that the 5.56mm cartridge was identical to the commercially available .223 Remington round. Here is their 31 January 1979 release, with some minor errors corrected:
With the appearance of full metal jacket military 5.56 ammunition on the commercial Market, it has come to the attention of the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) that the use of military 5.56mm ammunition in sporting rifles chambered for Caliber .223 Remington cartridges can lead to higher-than-normal chamber pressures and possible hazards for the firearm, its user and bystanders.

Tests have confirmed that chamber pressures in a sporting rifle may be significantly higher in the same gun when using military 5.56mm ammunition rather than commercially loaded Caliber .223 Remington cartridges, according to SAAMI.

SAAMI points out that chambers for military rifles have a different throat configuration than chambers for sporting firearms which, together with the full metal jacket of the military projectile, may account for the higher pressures which result when military ammunition is fired in a sporting chamber.

SAAMI recommends that a firearm be fired only with the cartridge for which it is specifically chambered by the manufacturer.
Additionally, SAAMI's Unsafe Arms and Ammunition Combinations Technical Data Sheet page states:

The .223 Remington is rated for a maximum of 50,000 CUP while the 5.56mm is rated for 60,000 CUP. That extra 10,000 CUP is likely sufficient to cause a failure in a chamber that's only rated for the "sporting" .223 Remington.

The .223 Remington and the 5.56mm NATO, when checked with a chamber ream from a reliable manufacturer of each, also have discernable differences in the areas of freebore diameter, freebore length (leade) and angle of the throat.

AR Hammer 09-10-2007 04:42 AM

Glad to see someone else is trying to get the word out!

97powerstroke 09-10-2007 07:45 AM

Thanks, now I know. Why is there no mention of this in any of my loading manuals? The only thing I could find in The three books that I have are about using the heavyer bullets in the higher twist rate barrels. 1 in 10 and higher. Speer, hornady and nosler have nothing in them about this. Thanks for the info.

AR Hammer 09-10-2007 03:50 PM

The extra pressure comes from the military using a 'Ball' powder instead of the IMR 'stick' or extruded powder Remington intended when they homologated the .223 Rem. round with SAMMI.

The military didn't have to check with, or register with SAMMI when they decided to make changes to the powder for their rounds...
And they didn't check with Arma-Lite/Colt or anyone else either!

The use of that ball powder is the reason for the AR-15/M-16 early reputation for jamming and undependability.
The early rifles were designed for .223 ammo, and the military ran 5.56x45mm ammo through them...
The ball powder gave a little more muzzle velocity and added to the long range accuracy,
Something the military wanted then, and is still looking for today... Accurate long range rifle fire.

An adjustable gas rate tube is a good idea of you are going to fire both types of ammo, (along with using a 5.56x45mm rated barrel~).

fmJK-47 09-17-2007 11:14 PM

the 556 has a thicker brass for durability, this increases the pressure of the military round over the civil. 223 Remington

ranger_sxt 09-18-2007 07:00 PM


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.

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