Im a little puzzled.  I hear alot of experts say that a 1/9 twist barrel is the most appropriate for a basic AR 15 rifle.  Everybody use's the 1/9 (bush, oly, rock, etc.,)  So WHY doe's colt (even the NEW M4) still use 1/7???  Do they think its better? is it cheaper to make?  I can't believe that colt w/ their name, would purposly put out an inferior rifle barrel to everybody else's.  WHY the 1/7 twist??

Somebody correct me if I'm wrong but I believe it has to do with the ammunition. In order to properly stabilize the tracer rounds the 1/7 twist was needed. I think I'm correct on this but I have been wrong before.
Jmpr90

It is to shoot USGI tracers.
cpermd

It is to shoot M855 & M856

Indeed, Colt adopted the 1/7" twist to stabilize the newer 62gr loads of the military, and to accomodate even heavier rounds in the future.  There has been some concern over throat erosion at the start of this very fast twist.  Colt does offer other twists, like the 1/9 in my 6721 Tactical Carbine upper.  This accomodates older 55gr ammunition that is preferred (by many, including myself) out to 200m.

I used to have a 6600 Match Targe HBAR that really liked 69gr MatchKings in the 1/7" twist, but it didn't really do very will with 55gr plinking loads.  The 1/7 seems to succeed with the heavier 77gr and 80gr projectiles that a lot of long range shooters like.  I think the 1/9" will do OK with 62gr, and works as a good compromise.  Bushmaster offers a pre and post ban 1/9" M4 upper which is as good as the Colt, even without the extra cuts in the upper receiver.

As has been stated, GI barrels use a 1/7 twist to stabilze the M856 tracer rounds.

Colt 1/7 barrels are closer to mil-spec. Hell, they are mil-spec. They are probably using GI contract barrels. All of my Colt 1/7 barrels shoot both 55gr and 62gr ammunition just fine. Accuracy-wise, I can't really tell a difference between the two.

Colt does use unlined 1/9 barrels on it's ARs that are intended for use as more dedicated target rifles.