I'm really likin the 300 BLK. I'm just waiting to start my first in that caliber. As an SBR though. DD M4 lower with an AAC 9" barrel. 5.56, I've got the seekins stripped lowers coming (that we've laughed about before) to build some completely unique rifles (to me and my collection). I liked "the Jack" that you built and painted. I'm really interested in something like that. Gonna have to check into some Bravo pieces.
The other calibers are interesting too, but I haven't ventured into the pistol caliber AR's yet.
Like I said at first. I've had target fixation on the 10 platform for quite a few years.
I'm a big fan of 7.62, but you gotta love them all.