For 16 inch barrels...
Having built and fired a number of 300 BLK uppers, I HIGHLY recommend Daniel Defense barrels for this caliber. $279.00 unless you can find them on sale. I have AAC, CMMG, Mossberg/Diamond, and some el cheapo brand I won't mention. Daniel Defense blows them all away for both function and accuracy.

For 8" - 10.5"...
I HIGHLY recommend KAK Industries Mr. Blond barrel(9inch) - $265.00. In these lengths I have PSA, and AAC as well. KAK blows those away for function and accuracy.