Here are a couple experienced guys...
https://www.youtube.com/results?search_query=nutnfancy+ar+15
"The Perfect AR-15" by Nutnfancy, 5.56x45mm; For short range (CQC), upper 16" barrel, Aimpoint 2mil dot for deep snow, Bushmaster; For one choice, most use, and longer range, 18" Stoner barrel mid-weight profile fluted, 7 lbs, DI, Griffin muzzle, 13" Troy Alpha rail, Chip McCormick trigger, Nikon BuckMasters 4x14" (med qual), Unamount Night force scope mounts, no scope covers, commercial stock tube, Magpull CTR or MOE stock, MOE grip, 8lbs 11oz.
https://www.youtube.com/watch?v=OJASJhcFQSc
7.62x51 preferred for combat rifle. With size, weight, resupply (POU), then AR-15 comes in.
DI vs Piston, DI for semi, piston for auto.
Duracoat dark earth
20" barrel too heavy, approaching 308 weight. 11lb. Less than 16" barrels take too much away from a weak round.
30 mil scope weighs more, costs more; 1" scope gathers light just fine.
Best AR 15 scopes, nutnfancy
https://www.youtube.com/watch?v=pEE59GKnk2k
Feb 2013: $200-400 med price. 50-300 med yds, < 20 oz for close to med range (even longer range), mid range doesn't need target turrets, 30mm is not needed unless night shooting, shorter is better, eye relief 1.5" excellent, field of view very imp
Weaver rimfire 3-9x32 AO 12oz, 11.3" OAL, 849318 usable on AR, 3.1 eye, 60" $220
Weaver 2.5-7x32, 10 oz, dual x reticle, or TMR, $150.
Weaver 849405 2-10x38, 11oz, 12.3" OAL, $160, 3.3-3.6, dual x, 70" highly recommended
Redfield Revolution 2-7x33 67085, USA, great eye relief 3.7-4.9", 11oz, 65" adj, $190
Redfield Revolution 3-9x40, 67095 12.6oz, $210
Burris 3-9x40 Fullfield II #200159, 13oz, 3.1-3.8 eye excellent, 50" adj, 13-33', $200, love em, no zero stop (mark the tube), great dials
Burris 4-14x42 FFL II, 200185, 18oz, 22-17, 40" adj $300 13" OAL, 3.1-3.8 eye
US Optics SN-3, 1.8-10x TPAL 24oz, $200
Nikon Buckmaster 4.5-14x40, #6453 18oz, good dials, 6.8-20' FOV, SF, 3.1-3.6 eye, side paralax adj, not great FOV, $230
Nikon M223s 2-8x32, 8486, 15oz heavy, 11" OAL, 3.8" eye, $330, big turrets
Burris sets the standard for mid-value scopes, clarity, high rec for night
Burris 2.5-10x30, MTACs have replaced TAC 30s, good scopes 80" adj, illuminated, 15oz, 13" OAL, $360
Burris 6.5-20x50 really like for long range 200199/200193, SPR POU, 19oz, super bright and clear 95%, down side 30" adj and 1" tube, have to shim the scope to get long range, $400
Nikon 3-12x32, 8488, 20oz heavy, 13" OAL, 60" good FOV 8.4-33, $450
Weaver 6-24x42 17.6oz, 14" OAL, semi target turrets, 3.1" eye a little critical, mil dot, 1" tube $340 849412
Bushnell Elite 3200 5-15x40, #325154, 19oz, outstanding dials, 90% light trans, 4.3 eye, mildo, 50" adj $390
Leopold 1.5-5x20, #54902, non-ball reticle, illuminated, 3.6-4.4 eye relief, 14oz, $500
Voretex Viper PST 6-24x50, EBR-1 reticle, 30mm, FFP, 4" eye, 23oz $900
Leopold 2.5-8x36, illuminated, Mark IV, dual knob 3-3.7 eye relief. $1100, 16 oz, not worth it unless you spend hours in the scope.
Leopold 4.5-14x50, #60005, M1 dials, 12.5" OAL, 22oz, $1200
Nightforce 2.5-10x32 NXS Compact 19oz, 12" OAL, illuminated, critical eye relief 3.7", great field of view 44-11', zero turret stops, $1200
Nightforce generally too heavy for longer range, unless you need extreme toughness. 30 oz.
Gun Porn at Gunnies: Best AR-15 for Under $1000
https://www.youtube.com/watch?v=mt9P0HTHojc
Good luck, shoot safe.