I thought all Colt’s were able to handle 5.56mm pressures. Is this not the case? This is on the model 6721 to be exact. Thanks for any response in advance.

I'm not very knowledgable about AR's, but I don't think the lower has any determination of whether it can shoot only .223 or 5.56 also. I believe that's the barrel chamber on AR's.

quote:

Originally posted by jimmy123x:
I believe that's the barrel chamber on AR's.

I agree. And it isn't the chamber pressure that sometimes causes problems with shooting 5.56 in 223 chambers, it's the case and bullet seating. (As I understand it.)

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.

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.

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.

6721 is a 5.56 chamber.

.223 and .222 caliber marked lowers are generally meant for the overseas market.

Italy and South Africa being an example

18Z50

.223 is often used to make it sound less military like. Your rifle is capable of shooting either .223 or 5.56mm.

quote:

Originally posted by Zero1911:
<snip>
The coveted lower is the one that reads Tactical Carbine but even that one has a 223 marking on the lower.

I can confirm that ...



However my M16 is stamped 5.56 on the receiver;
so I think its just the difference between a commercial and LEO/Military receivers

'73 SP1