Good barrels, good triggers, proper scopes, free floating handguards, solid stocks, and well developed loads. That’s been the consistent recipe for me to assemble literally hundreds of uppers achieving sub-MOA results. Little tricks and traps here and there seem to make life easier along the way, but even without these, the above list will achieve the goal.
I don’t buy uppers any more and have not in many years, but Compass Lake, White Oak, Mike Milli, and GA Precision would be the only folks who would get my money if precision were the objective of the day (which is typically the case whenever I’m building an AR). For barrels, I’ve never been disappointed by a Shilen, Craddock/Bartlein, Proof, Krieger, or even Black Hole Weaponry barrel. I prefer Geissele Hi Speed Nat’l Match 2 stage triggers, but have been very happy with Bushmaster Nat’l Match 2 stage triggers, and content enough for plinking use with Geissele G2S, Rock River Nat’l Match and Varmint Match, and Larue MBT2’s. For Single Stage, Elftmann. I use AGB’s for most of my builds to give control to the shooter, and H2 buffers to smooth out the recoil impulse. For a Carbine stock, Magpul UBR, for fixed stocks, Luth-AR, Magpul PRS, or XLR. I prefer long handguards for long bipod span, favoring 18-20” handguards when possible - but I highly favor integrated arca rails for some applications, which I’ve not yet found offered on 18-20” rails (but on 16” guards).
Frankly, given a rifle built with the original list I offered above, chambered in 223/5.56, if I load 27.3grn of Varget under a book length 50grn Vmax, if it doesn’t shoot sub-MOA, I know something is wrong with the rifle.
