Here goes my nose out further than it should.
1) Thank you for your service. After 22 years Army Reserve, and not liking the concept of light, handy, easy to shoot service rifles and opting for minimum 12 pound .308s, I woke up and saw where they weren't all that. I'm finishing an AR build this month.
2) What you want it to do should be spelled out. No sense getting an RX8 when you actually need a Cherokee and a trailer. Specified features make the gun do some things better than others.
3) Nail down the level of accuracy you need, the gun builds on that, even if you decide to buy complete. How much MOA do you need? Milspec is 2MOA, prairie dog spec is 1/2MOA. Everybody wants to own a 1/2 MOA rifle, it's a lot harder to get out of a 16" carbine. Setting the MOA means then choosing the optimum barrel length, and everything then cascades into a prioritized order.
Picking and choosing parts wont' work well, wastes a lot of money on bling, and shooting it then leaves some questions, like why was $1500 spent on a collection of parts using a standard military chrome lined 2MOA barrel? Not even SOCOM does that, but pics are posted up almost daily in the AR world.
Barrel, bolt, and caliber - yes, there are choices beyond 5.56. Then upper, for the A3 flattop, optic, then furniture, and last, trigger.
Do it backwards, spend $500 for a drop in trigger and free float quad rail, and you have a 2MOA military barrel with great trigger feel and 48" inches of rails that even Knights Armament, the contract supplier, says won't do the average shooter any good.
No option on the face of earth makes a gun shoot twice as good as the barrel already can. If it could, we'd all buy shot out surplus barrels, add $1200 worth of sniper stocks, quad rails, 3x9 red dot scopes, VFG's, and shoot 600M with 3" groups.
Can't be done, but look around, pics are posted daily in the AR world. 22" dub rims on the family 4WD truck aren't the best choice hauling a load of sheetrock up the unimproved drive 1/4 mile to the vacation cabin in the Ozarks.
Here's one example: Shoot deer with an AR. 2MOA work. Best caliber for that in the AR is 6.8SPC, it has the most on the market, and ammo is even on the shelf locally. Best barrel to do that is the ARP Socom 16", midlength gas. A3 upper to mount a red dot or 3X scope, most shots are 200 meters. Fixed A1 rifle stock, rifle handguards, TD battlegrip, because it was the smallest available in Foliage Green. Issue trigger with adjustable set screw, feels just like the Rem 700 with a bit more pressure.
Probably more accuracy than I need, but still under $900 without optic. The stripped lower was AGP, $79, the stripped upper an LAR blem, $48. The barrel came with headspaced bolt, cam pin, firing pin, all nitrided, and a matching nitrided bolt carrier added in. Very slick tough finish, all my user knives have it and will NOT scratch up after years of use. When I filed the burrs from the gas block dimple, the file would not cut anything other than the base metal. No marks even tho I was sawing on it like a fiddle.
Spec what you want it to do, work down the list, don't jump ahead. You will get a purpose built rifle that actually does a better job than a pile of ubercool parts recommended by fanboy geeks who's claim to fame is that tag line on the pic they posted, "Haven't shot it yet, it's been too cold/wet/hot/busy."
Combat Arms knows its all about the shooting.