Thanks to the information from Aqualung, I finally got my 300BLK finished. I bought a complete upper from Brownells which arrived today. When looking at it, the Upper itself has the same "Keyhole" proof mark that my AR556 has, but the barrel is a mystery. I did some searching and couldn't find anything on the maker; so my question is, does anyone know who makes the barrels for Brownells? The upper received good reviews and I know about Brownells' return policy so I'm not concerned about quality, just curious on the maker. Thanks in advance!