Subsonic accuracy really depends on two things; bullet design and twist. In my 8" with a 7 twist, I have a really hard time getting good groupings with match designed bullets like most are using. it's not that it's a bad barrel, and even a 7" twist is more than most I think who are using 8s. However the same barrel shoots and easy half moa with a supersonic load of 110 vmax. I think there's just too much procession or yaw in the slower rounds. I do think it is quite possible with a shorter, more blunt, and high drag projectile much like we're used to seeing in low speed platforms, ie: pistol calibers or. 22s.
This begs me too ask the overall practicality of the blk. My best reliability and accuracy, not to mention terminal effect, comes from a lighter super sonic load. Why then not stick with what we already have, being the 5.56? Knock down power? Then an AR10 or AK all ready fits the bill. The prospect of running the AR15 platform with subsonic suppression was a long term goal for many of us, but now that the golden goose has hatched, I'm finding myself at a loss. It's a very fun platform, no doubt, but for myself and those who have ever had the unpleasantness of having to kill something with a sub sonic projo (more specifically a.30 sub projo) the effects can leave much to be desired. Short of a dome shot it takes effort and close distance.
Having just built a nice ak47, I'm selling my upper as we speak. Is a very viable system with a 110-130 range bullet pushed fast, but for the cost of ammo and versatility that the new AK adds to the family, I'll give up my beloved upper to help me with other goals.
In short, yes the accuracy can be achieved, but easier done going fast. I hope others chime in with better success stories, but for the level of suppression actually gained, I'm all for my suppressed mk18. The longer 16" blk uppers provide better suppression, reliability and supersonic MV. However let's be honest, what's the point of you don't NFA that bitch?