The items your brother installed on his may help him to shoot it better, but they don't do anything to make it more accurate. That's up to the barrel alone.
Most accessories simply eliminate bad influences on the barrels inherent ability. A milspec AR will have a sling swivel attached to the gas block, tighten up on it and it will change the point of impact, sometimes dramatically. Free float tubes are installed to eliminate pressure on the barrel by attaching the swivel to it, not the barrel.
While many would call that an accuracy improver, it isn't. It's really an inaccuracy eliminator. Big difference, as putting one on a 2MOA milspec chrome barrel makes it a 2MOA milspec chrome barrel - not a stainless 1/2 MOA match.
Forget about brand fan recommendations, you need specifications:
At the minimum, a premier stainless match 20" rifle barrel, free float smooth tube, A3 flattop upper, fixed stock, and anybody's trigger with a travel adjustment screw. Add one really good optic nearly the cost of the gun, and very few would complain.
Avoid chrome lined military barrels, proprietary billet receivers, adjustable M4 stocks, expensive modular grips, and don't even think about shooting cheap surplus or import fodder. Accurate guns have to shoot accurate ammo, premium loads earn their price by keeping the deviation in speed to an absolute minimum. Many loads improve accuracy of the system by another 1/2 MOA compared to the cheap fireworks some stuff in a magazine.
It's simple to surf the web and view all the varmint AR's out there, and not a bad idea to survey them and see what the specifications are. You should consistently find the barrel and painstaking assembly as the two major items, not fad parts or brandboys arguing nuances over their one gun sample data base. It also won't hurt to read posts on long distance shooting forums to get an idea of who has a good reputation. There are some makers with splashy web pages and great prose, but the reality isn't there.
If you want an accurate shooter, it's not about X vs. Y, it's about stainless button rifled vs hammerforged nitrided, or standard scope construction vs. first focal plane reticle. At those levels, the brands sort themselves out real fast.