The problem is that most subsonic rifle ammo is made with very heavy bullets. 80 grains is about the heaviest I have seen for the .223. To keep it down to subsonic level it will be a very reduced load. Reduced loads have their own inherent dangers.
A 1/7 bbl should stabilize an 80 grain well enough. 80 grain bullet at subsonic velocities will be anemic at best.
80 gr subsonic will not cycle an AR-15. I have played with a friends AR in .300 whisper. Quiet, subsonic, abt 2 moa at 50.