and it's hard to get a .223 subsonic stabilized in a 1/9 barrel, like in most AR...
I tried it in my M4 with supressor, using 80grains molycoated bullets and shooting rounds 1 by 1, since it would not cycle the bolt... but frankly the sounds suppression is not worth it (mostly because .223 supressor are made to suppress supersonic gaz expansion, and you will still hear the supersonic crack, but you shot will be very hard to localize) compared to a standard 22lr ammo supressed in a bolt rifle or semi auto, which make NO more noise when shooting than a airgun (pellet gun).
you could stabilize a subsonic 55 grain bullet in an 1/9 barelled AR, but than, the ballistic efficency of this bullet would be exactly the same as a 22lr, and it would still make more noise..
If you want something really stealthy, go for a good 22lr supressed.
Now, I heard about 300 whisper etc, but never tried it. maybe it's the best way, if you can afford it. Here, there's no such way.