[Manly Warrior]

I would assume it is a 223 rem chamber, and therefore commercial 223 rem ammo would be the safest. 556x45 NATO ammo is loaded to a max of 65000 psi, same as 223 rem., the difference is that the pressure testing methods for each are slightly different-223 rem is tested in the chamber itself, while the 556 is tested at the forward portion of the chamber called the throat or leade The 556 pressure if tested as a 223 rem would nominally be higher). Also, headspace (the distance from the bolt face to the reference diameter of the neck, where the cartridge is supported) is longer for the 556, so 223 rem rounds are normally slightly short; excessive headspace can be the cause of case separation and violent expulsion of high temp/high velocity propellant gases-which are quite dangerous to say the least.

All that being said, I always recommend using the exact cartridge the firearm is chambered for. Assume 223 rem. unless you have the chamber measured by a gunsmith to verify its internal dimensions.

God Bless and MOLON LABE

[Manly Warrior]

Correction: 556 is 60k psi, 223 is 50k psi-thinking of another similar pair...

[Jed Eckert]

The vast majority of genuine Colt AR15s were in fact chambered for the 5.56 to avoid liability issues since the .223 Rem could safely be fired in the 5.56 chamber.

To add to the confusion most were marked .223/5.56

This would indicate either cartridge can be fired. If the barrel is marked only .223 Rem then caution is in order.

Nevertheless it doesn't hurt to check with Colt's customer service.

In the past I've found them to be very helpful.