Definitely “quality ammo” is key. But, you don’t need to reload yourself to get sub-MOA results. There are a TON of quality companies out there kicking out match grade ammo at reasonable prices.
I really like Defender for example.
Agreed, to an extent. The difference, for me anyways, between sub MOA, and MOA, is .1 grain of powder. I think quality ammo is luck of the draw. The cost of that ammo is super expensive and trying them "all" is akin to building up your own load. The choices in bullets are a lot greater as well. Granted, the cost of building a quality reloading rig, learning, and getting all the components, is well up there. I believe, maybe 1 out of 10 times, can a high quality ammo compare with what you can make on your own. I also know that this is kind of daunting for someone that's new and wants to build an accurate AR.
Backfire is a youtube channel I enjoy watching. He's not sponsored by anyone, buys all his own stuff, and posts honest reviews. He had a Sig Cross review a week or so ago, (well, an updated review after a year of use), and had "ok" results with all the high end ammo he tried. Then he had some quality reloads and turned those "ok" results into "amazing" results. Every rifle is different. One round that works "amazing" in one rifle, becomes "ok" in another.
I also don't want to derail the conversation, so I apologize to the OP. As has been said, the most important parts of building a sub MOA AR has already been covered. The barrel is at least half the equation, trigger another 30%, and everything else after. IMHO anyways. Also, for 223/556, definitely a Wylde 223 barrel.