Those arm braces sure are handy, do you happen to know the manufacture of that one. I just finished up a braced 9mm AR pistol. It's now one of my favorite firearms. I'm really thinking about building a 300 BLK upper in 10.5 to go with it.
Not to stray to far off topic, but I watched a YouTube video discussing differences, and similarities of 300 blackout, with 7.62x39. Both very good rounds, for a variety of purposes. Of course John I know you have a love for both. Just throwing out there that I hadn't really seen a head to head comparison of the two.The 300 what'cha'ma'call-it is a pretty decent round. Least for what I use it for.