[Doomonyou]

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.

(Ironically, given the nature of the on-line confusion exhibited by .30 caliber shooters, no similar SAAMI advisory is given concerning 7.62 NATO beyond the fact that those who have rifles chambered in "308 Winchester" shouldn't attempt to shoot cartridges marked "7.62x39" or "300 Savage." Well, duh!)

However, the estimable Clint McKee of Fulton Armory, has thoughtfully provided a brief monograph, The difference between 5.56mm and .223 Remington chambers in the AR-15®-type rifle, which explains this issue in greater (but quite readable) detail.

Olin's Winchester Ammunition site, in 2001, addressed this matter as well, in a concise monograph by Paul Nowak, and Randall Rausch has a number of excellent technical documents, with graphics displaying the differences between the two cartridges, available at AR15barrels.com. Of particular interest are headspace and reamer dimensions.

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http://www.thegunzone.com/556v223.html

This article deals with "Real" guns and ammo. Not some toy. I'd like to see the actual claim tested. (from a safe distance.)

[Myrddin]

There is a difference between .308 and 7.62x51 NATO. I learned that the hard way in my Remington bolt action "Police" rifle. The throat is narrower in that rifle. .308 only. The NATO round slam fired. Fortunately, I had the muzzle pointed downrange when I was closing the bolt. Lots of gas in my face and unanticipated recoil. The good deal I got on the NATO ammo wasn't so good after all. I paid too much for the rifle to risk damaging it or being injured by the wrong ammo. C'est la vie.

I only shoot commercial .223 in my Savage bolt actions as well. The real AR15 is fine with NATO or .223.