I personally don't see a purpose for the .300 blackout, unless I were a covert operator that wanted something under 1100fps. For deer, there are better "medium caliber" choices in the AR-15 platform, for coyotes, there's no need for 125grn+ bullets at all, and the slow speeds attained by the Blackout is just silly. 20moa drop by 500yrds with a 125grn pill? WTF? Why?

.223rem all the way. I run 9" twists, usually 50-55grn bullets for coyotes. Never felt the need for a heavier bullet, although I've ran 69-75grn pills for them. 8" twist if you want to run heavy bullets out of a shorter barrel. The only heavies I shoot are from 24" tubes, so getting the velocities high enough to stabilize isn't a challenge.