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Wow! Thanks! Just made the barrel world more confusing with choices! In all seriousness though that is alot of great info! Guess i got alot of reading and deciding to do. I really like the AR15 sport because its out of the box ready minus the sights and sling swivel. I dont have the means to drill and install a FSB so im looking for already assembled. Id love to be authentic with 1x12 but more ammo options with 1x9....decisions decisions!
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Glad I could help!
I'd say the best way to find a barrel would be to go by what matters the most and move down.
Since you are looking at the AR15Sport and Brownell's products, I'm assuming that you are looking for a barrel that looks correct on the outside and that that means correct gas block diameter.
You also mention you don't have the ability to drill the barrel for an FSB. If there is a barrel you really like, don't let that stop you as you can get that done (albeit for an additional charge). If there isn't a must have barrel and you aren't doing an all-Colt build then this probably isn't the most cost effective option for you.
From there you have to look at extension / feed ramps, twist rate, finish, and chrome lining/finish.
In my build, I wound up getting an offer of a Colt barrel assembly, but before that I was looking at many of the options you are doing. going with the above assumption of external appearance, I'd avoid a nitride finish as they are pretty much jet black and will contrast sharply from the other parts (unless you are doing a black rifle instead of a grey).
Chrome lining is up to you, and of the list that are assemblies all are chrome lined except the 1:9 AR15Sport (I'm just not sure if that one).
Extension wise, I've read that it is best to have them matching the receiver, but M4 ramps with a rifle receiver is still functional.
With all that in place, it really does seem to teeter on your choice of twist.
I am fine with 1:12 because I mostly shoot 55gr anyways. If I want/need to shoot heavier/longer stuff, I have another AR with 1:8 and another rifle with a 1:7. Based on the assembly cost, I think you have a pretty simplified list:
-1:12 Twist: Brownell's. (Cheaper than the AR15Sport 1:12 offering and with rifle ramps))
-1:9 Twist: AR15Sport
-1:7 Twist: Look at the one BoyWonder just asked me to add.