There must be some top AR manufacturers doing this then. Can you give me a couple examples? KAC? DD? Noveske? JPE? BCM? Lanxang? LWRC? GA Precision? HK? LMT?
Exactly. If the key to accuracy were upper to lower fit, and it's that easy to achieve, companies like JPE and Lanxang would be doing it.
I've got a JPE that I special ordered from them years ago. Hands down, the most accurate semi-auto .308 I've ever shot. I've never noticed the upper to lower fit to be anything different from what anyone else does.