Lot's of AR-15's sadly are just very poor loose sloppy builds!
The upper receiver, BCG, M-lok type free float with quality barrel nut, and barrel make a world of difference.
I have some competition 15's and I use quality drop in triggers. I also use Caldwell Lead sled rifle holder to really dial the sights in. It take out the human element. You need all the same brass for consistency. IMO 75 grain pills are the max weight. The case and mag length starts putting the bullet too far down inside. That's why they came up with the Valkyrie.
I was able to buy boxes of 5000 very high BC 62 gr HPBT and dialed them in at 3120 fps out of my 20 inch with 11 degree crowned match grade barrels using commercial brass and about 27 grains of CFE or LeverRevolution powder.
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CMC AR-15 Tactical FLAT 2.5lb Single Stage Drop-In Trigger Assembly