I am planning my first build and need some honest advice. This will be a hunting gun, and while I don't need benchrest accuracy, I want it to be as accurate as I can make it. My question is, should I get a matched set upper and lower-RRA is what I was looking at-or can I mix and match to save some $ and still build a 1-2 MOA gun?
The upper/lower isn't really what makes it accurate, it's the barrel.
Barrels launch the bullet, they need to be as close to parallel as possible inside the bore, tuned to reduce harmonic vibration at the muzzle to almost nothing, and shot with carefully constructed ammo with as little variance in speed and footpounds as possible.
HIgher velocity gives a flatter trajectory, which means less drop calculation, less time in flight to be off, and more power to resist windage. Having the muzzle not vibrate is like holding a garden hose, three feet from the nozzle, it sprays with little control, right at the end, you get precision. A tuned length barrel uses shape, mass, and just knowing where the node is and cutting it there - for that caliber and handload. You can butcher up a lot of barrels to learn that. Consistent ammo means you don't get fast or slow bullets with differing bullet drop, spreading the group size.
A quality barrel, mounted to a squared receiver, equipped with good sights, and shot with good ammo, should get you under 1MOA. From there, you could just add a free float tube and nearly any lower with a stock duct taped on, adjust a milspec trigger with a travel limit screw, and be done. Despite all the marketing, sniper stuff sold for AR's is based off bolt action space gun tech, it's 1/4 MOA stuff that can't be appreciated until the barrel and ammo can do it. Beaware, it's barrel, matched bolt, proper assembly, sights, then trigger, etc. A 2MOA barrel with free float, $250 trigger, and fancy Magpul stock is a 2MOA shooter still. Nonetheless, many here will recommend painting stripes on a Metro and racing it at Indy.
That barrel may be stainless, or nitro carburized, hammerforged, or buttoned, but it won't be just any cheap milspec chromed alloy stick, that's a guarantee of 2MOA with issue ammo, that's all.