Yes, digging into it, I plan to study this out till January then make a decision. great site so far.
That's good, there's a lot of crap out there and the more you know, the easier it is to shift through the crap.
As an FYI, 'tightness' doesn't matter one bit in the ARM4 family. Neither does 'fit and finish'. These aren't 1911s. My priciest and most accurate rifle does not have tight upper/lower receiver fits and the stock isn't that tight on the receiver extension. It shoots 0.5MOA at 100 yards with MK262 ammunition. Pretty damn good for a semi-auto.
I'd worry about getting a quality foundation, keeping in mind:
1. the barrel - barrel steel, twist rate, gas port dimension, chamber, etc. all need to be in spec as they have a direct relationship to the function, reliability, and longevity of your carbine. Brands I trust are Colt, Daniel Defense, Bravo Company, Lewis Machine Tool, Centurion, Noveske, Knights Armament, and recently Palmetto State Armory has been decent.
2. the bolt/carrier - while you can swap factory units out for better ones, these also have a direct relationship to the reliability of your carbine. Brands I trust, and have thousands of rounds through, are Bravo Company, Daniel Defense, and Lewis Machine Tool.
Rails, lights, optics, triggers, stocks, etc. can all be changed later and you'll probably go through multiple iterations until you are happy. But you can rest easy if you know that your foundation (barrel, bolt) is a good one.
The MK12 I mentioned
The target...before someone calls me out