Factory Colt AR-15 barrels have always been chambered in 5.56 NATO regardless if they are marked .223 on the lower receiver. The difference between the two is in the throat of the chamber. 5.56 chambers have a longer throat than .223, which allegedly/potentially could cause an overpressure situation when shooting a hot 5.56 load in a tight .223 chamber. But there would not be any issue going the other way.
Posts: 1718 | Location: WI | Registered: December 29, 2012
Of course lower has nothing to do with it. I have not seen a colt barrel (real one) that wasn't marked 5.56. I'm sure given the history and volume they exist, but I've just never come across one. Mine is marked 5.56.
“So in war, the way is to avoid what is strong, and strike at what is weak.”
I believe the Colt 6721 rifles have always had a lower marked .223. Total guess but this gun originated in the 1990s around the time of the federal and state assault rifle laws. Maybe Colt thought the HBAR and marking it .223 could help it avoid being banned because that made it seem more like a civilian sporting rifle.
The 6721 has a 556 NATO 16” 1/9 HBAR barrel. The lower, handguard and bcg have varied along the way. The coveted lower is the one that reads Tactical Carbine but even that one has a 223 marking on the lower.
Posts: 17 | Location: NM | Registered: May 30, 2020