I'm not even remotely an expert on the AR. I assembled my own lower. That doesn't mean much. I learned some stuff from it though. I can't assemble my own uppers because I don't have the appropriate tools.
However, I can vouch for tp555. He made my 300-221 barreled upper and 223 barreled upper. Sadly, I haven't shot the 300-221 yet. But the 223 performs like you would not believe. When I had the work done, I included the bolt I intended to use for the upper in question for head spacing on that bolt.
Also a bit of sage advice I got from him (and I see it referenced in this thread) is that it is far easier to make the brass fit the chamber than the other way around. The 300-221 is a wildcat. JD Jones notwithstanding, there are no official dimensional standards for the cartridge. About all you can count on is the SAAMI spec pressure for a 223 case if you go ahead and form your brass from that.
Fit the brass to your chamber. You should be good.
You know, barrel blanks are not all that expensive. An experienced barrel maker like tp555 can put together an upper for you for a fair price. It won't be some cheap job that you find in an off the shelf AR. You will have an upper with a custom barrel suited to your needs. Ultimately, it is the barrel that determines how well your rifle can shoot.
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