Re: advice wanted---carbine build
The first question you need to ask yourself is "what will I be shooting". The bullet type you plan to shoot the most (which determines it's weight and length) will tell you what twist rate you need. Under/over-stabilization will kill your accuracy.
If all you ever plan to shoot are 55gr or 65gr FMJ ball rounds, you are fine with a 1:9 twist. If you plan on using 62gr penetrators you will want to step up to 1:8 or 1:7 (the bullet is longer and need a higher twist to stabilize).
I run 1:7 on all my AR's, regardless of barrel length because I use a wide variety of ammunition, and it just gives me more versatility. Yes accuracy does suffer a bit when using the lighter stuff, but not by any noticeable measure from what I can tell.
Once you determine your twist, pick a barrel material. I suggest stainless for accuracy and consistency.
Contrary to what you might hear, stainless lasts just as long as a chrome lined bbl will. That is to say you will get about the same number of rounds out of each before it's time for a new one. Yes the SS is softer, but it starts it's life at a higher accuracy level than a chrome lined one. By time both are worn out they will both be capable of the same level of accuracy.
So the decision ultimately comes down to a choice between an <span style="font-style: italic">extremely accurate bbl that will turn to crap in 10,000 rounds</span>, or a <span style="font-style: italic">moderately accurate barrel that will turn to crap in the same 10,000 rounds</span>.
Profile I'll leave to you. Naturally the heavier the better (in terms of accuracy), but realize that going heavy makes things... heavy.
I have a 16" SS SPR-profile bbl in 308win on my LR308 that is heavy as hell as it is (it feels like an HBAR). I can't even imagine how much worse it would have been if I had gone with a heavy or bull barrel option, or had another 2-4 inches on it.
I like gov't profile bbl's the most though. It seems like a good balance between weight and accuracy.
So you have your twist rate and bbl material selected. Next is to determine bbl and gas length. You said you want a 16" so I'm going to say get a bbl with mid-length gas port for reliability. Carbine length gas on a 16" bbl is hard on an ar. By going to mid-length gas you keep the same distance from the gas port to the muzzle as you do with a 14.5" bbl with carbine length gas. The overall effect is lower felt recoil, less wear, and higher reliability.
The last thing to really worry about is the bolt carrier group. Do yourself a favor and get a chrome carrier with an MP tested bolt and upgraded extractor. And going one step further, make it an M16 carrier. The heavier weight will reduce felt recoil even further.
You add all these things together... and you can have one hell of an accurate AR.
If you're not up to building the upper yourself (you'll need some tools like a vise, ar15 upper vise block, armorers wrench, torque wrench, punch set, etc to do it all yourself), the best bang for your buck will be a complete upper from BCM, assuming they have what you want in stock (because they always sell out fast).
Noveske, LMT, and others make great stuff as well, but they tend to be overpriced in my opinion. The choice is yours of course. I know if I had the money to blow, I'd have a noveske AR just so I could put it through all sorts of hell and make all the jealous fanboys cry.
