The 300 BLK is alot of things, but it is not an inherently accurate cartridge like a 308 Win.
The best accuracy that I have seen from the 300 BLK is from the 125 AAC / SMK loaded with N110 and a 205M Primer. (3/4 MOA @ 100 yards)
I would set 1 MOA out to 300 yards as an accuracy goal.
I'm sure that Robert will chime in with some specifics.
He will likely post up the MIL testing protocol and the 300 BLK test results.
I agree. I shoot (2) bolt guns in the Blackout and have never used the cartridge in an AR - yet.
Of course it all depends on the bullets and load combination being used. When shooting supersonics, if you can maintain (5) shot groups that average .75 MOA @ 100 yds. using match grade bullets then you're doing well. I've had plenty of individual groups that have been far better than that but my average over the course of a shooting session will invariably end up being around .75 MOA.
Subsonics are another thing all together. If you could maintain an average of 2 MOA or less @ 100 yds. I'd be happy with that. I don't have a great deal experience shooting subs but I've got a Hornady 90gr. XTP load that averages 1.35 MOA @ 100 yds. that I might be able to tighten up a little with some experimentation with primers.