Recommend me a 20” sub MOA barrel

[Klem]

I agree, Bartlein makes superb barrel blanks but they don't do AR drop-in's. If you want a Bartlein then you have to source a gunsmith to finish the blank. And not just crowning and chambering, barrel profiling and gas port drilling as well.

Here's the page with their AR policy. The excuse for not selling drop-in's is arguable but to be fair, they decide what work they do and what they sell;
https://bartleinbarrels.com/ar10-and-ar15-barrels/