Originally Posted by Quentin
Of those three I'd go with the Colt 6920. It's an excellent duty AR, lighter and better than the heavy 6721 and ArmaLite. Now Colt also sells a lightweight (and I think, superior) version of the 6920 worth looking at, the 6720.
ArmaLite makes a decent midlength AR but they're so darn heavy. It is important to point out that they aren't the original ArmaLite that developed the AR-10/AR-15, that company went under long ago. Eagle Arms bought the name and the rights to "AR-10". Eagle/ArmaLite have done good things, including developing the first midlength gas system everyone wants today.
I used an ArmaLite HBAR upper for my first AR but sold it to get a much better Daniel Defense LW midlength upper.
The Colt 6720 is my choice of carbine until something demonstrably better comes along at a reasonable price.
The Colt 6721 has a heavy profile barrel with a 1 in 9 twist.
The Colt 6720 has a light profile barrel with a 1 in 7 twist.
The Colt 6920 has a government profile barrel with a 1 in 7 twist.
Here's my take on the issue:
1. Heavy profile barrels may be better for tighter groups as the barrel heats up, but the difference is marginal. If you heat up your barrel enough for it to have a significant effect on accuracy, it generally means you're in deep trouble and an inch either way of exactly where you were aiming means squat.
2. Government profile barrels are fat on the end and skinny under the hand guard. It permits you to mount a grenade launcher, but most of us don't have grenade launchers and few, if any, of us have any 40MM grenades.
3. The light profile barrel does heat up quicker, there is some dispersion or stringing of shots as a result, but it's a carbine and most of your shooting is going to occur at room-to-room distances, so a little shot to shot dispersion amounts to a lot of nothing.
The real question is this:
Do you want a target carbine?
If so, then go with the 6721.
Do you want a M4 look-alike?
If so, then go with the 6920 (or 6921 if you are willing to get the tax stamp).
Do you want a light weight defense carbine that is plenty accurate and quick to move from target to target?
If so, then go with the 6720.
The number of perfectly good carbines that were ruined by adding unnecessary pounds of weight to them with unnecessary crap is astonishing to me. I try to keep it light and easy to point because, from experience, heavy weapons suck if you actually have to carry them and/or use them.
If you don't have to carry it anywhere, then it doesn't matter. If weight is not a consideration, then get whatever you like.
I've owned all three. I sold the other two and I still have the 6720.
Apart from the barrel profile and twist rate, there's no other difference between the three apart from cosmetic features.