Just thought I figured it out.  So are our current M16A2 5.56mm Marked or .223 marked?
AR15's are .223 marked, so that's the difference in chamber and in barrel between the military and civilian? Although we have the same twist rate, but military M16 uses a more"loose" round than the civilian AR15s?

So is M855 a .223 or 5.56mm ammo? ss109 is a 5.56mm ammo, so it's dangerous for the .223 marked AR15 to shoot with? If it's not true, then how can people claim it's dangerous to shoot 5.56mm military ammo out of a .223 barrel?

If you have a millitary spec chamber, "Like a Bushmaster", then you can safely fire either one in it. Don't fire a 5.56 nato round through a SAAMI chamber tho. As far as the difference I'm no expert, but I believe the 5.56 is slightly bigger in width? Someone correct me if I'm wrong, OUT!

Your getting confused in that all Colts and Bushmaster AR-15s are marked 5.56 and cut w/ a Mil-Spec 5.56 chamber.
The problem arises when firing 5.56 mil-surplus rounds in custom (tight).223 Rem chambered guns as the 5.56 round is slightly longer in the neck and over-pressure problems could occur.

Hope this helps.
Mike

Does any one have the 5.56mm cartrage dimentions or what reamer bushmaster uses.I can find .223 but no 5.56. [:\]

So, the AR15s MAY be .223 marked, but their chamber and barrel is still cut and milled according to the military 5.56 spec?
I'm not confused about the .223 and 5.56 ammo,
I'm just confused about the AR15s marked .223, not 5.56, although they claim they have military specs.  

Then the 5.56 and .223 difference only is a concern when you are shooting a remington 700 marked .223 or any other bolt action rifles?

here, this has got to help.

HUNTER FROM ARGENTINA
OUT.

oh and BTW YES, military contract ARs (Colt, bushmaster, Armalite) WILL fire 5.56x45 without a problem, i know it is marked .223 in some of their receivers, but the barrels and chambers are milspec therefor 5.56x45.
HUNTER FROM ARGENTINA
OUT.

Most AR15 rifles will shoot 5.56, as long as the barrels are stamped 5.56 NATO or somthing like that.  Many lower receivers are also stamped with 5.56 NATO or .223 Rem or somthing like this, but this is NOT what you should go by in selecting ammunition to fire.  Look at the barrel.  Because of the modular construction of the AR15, a used gun may have non factory upper.
Bushmaster has both a 223 and 5.56 stamp on the lower receiver, but only 5.56 NATO on the barrel.

Most Varmint guns and other non-AR15 rifles are chambered for .223, and the barrel stamping should reflect this.

Use .223 in a .223 barrel only, but use either .223 or 5.56 in a 5.56 barrel.

.223 is NOT mil-spec, but can be shot through mil-spec chambers.

5.56 brass is not stamped with .223, as it is a different size cartridge.

Shooting 5.56 out of a .223 stamped barrel can be dangerous!

The cartridge cases have identical specs, the SAAMI chamber is different then the chamber for the M16 rifle.  M16, like all autoloaders needs a little more room.

Any safety issues are imagined.  (Unless you reload and drink at the same time.)

The .223 Remington cartridge was adopted by the US Army and because of NATO metric standardization was nomenclatured 5.56mm.  Probably because it sounded good.  .223 and 5.56mm bullets are, of course, .224" diameter.  They're the same bullets.

-- Chuck

Go to
www.clymertool.com
work you way in to the reamer database, select the 223, print, select 5.56 NATO, print it, lay the prints to set box's side by side. Notice that the 5.56 is only 1-3 thousandth's larger in a few areas and the 5.56 has a sligthly longer throat. The 5.56's throat will reduce initial pressure but cost in accuracy as opposed to the 223 (shorter throat and tigher chamber). I personally would not get excited, especially firing 5.56 in a 223 chambered bolt gun.

Is there any difference in chamber pressure between the 2 rounds? I thought I saw a post here the other day that said the .223 generated approx 40-45,000 CUP, and the 5.56 came in at 60000 CUP. Any truth to that? Is 5.56 NATO a 'hotter' load than factory .223 on average?