I shot iron sighted AR15's in NRA Highpower Competition. Back when I was shooting, Armalite, Bushmaster, Rock River were the most common AR's on the firing line. And believe it or not, shooters with stock box National Match versions of all of these brands won Major events, such as Civilian Service Rifle champ.
Barrels were what you wanted, NM Armalites were Wilson match barrels, and I suspect, Wilsons were used on the other brands. You could pay more for Kreigers, Shilen, etc. And they all shot outstandingly. Chambers were always "match" chambers, the Wylde chamber was common. At the time, two stage triggers were versions of the Garand/M14 trigger mechanism. Now I see drop in modular triggers. You can pay more, and buy a more expensive trigger with a lighter pull, which is all to the good for a target rifle.
At some level of quality components, AR's will outshoot the owners, regardless of the brand. AR's are not like 1911's, which the most accurate have to be hand fitted, and Les Baer assemblers hand file the barrel bushings tight. You can assemble a AR15 from quality parts, the thing can rattle, and yet shoot dime sized holes at 100 yards, as long as the ammunition, barrel, and chambering job are right.
I earned my Distinguished with an M1a, and to make a match rifle out of an M1a, that rifle had to be built by someone who knew what they were doing. Bedding took a lot of hand labor, so did the unitizing the gas system, getting the correct barrel tension on the upper band, etc.