I would go .22 or any Whisper long before I wasted a dime on subsonic 223.
You'd have to bring the round down from about 3,000 fps to about 1,000 fps.
Stabilizing that would be near impossible.
As I understand it the problem with most subsonic rounds is that there is a lot of room for powder to flop around in. Making the powder burn random at best,
Subsonic 223 would have a lot of room for powder to flop around which can be solved with fillers like cream of wheat but I'm not sure if fillers and suppressors mix all that well.
As I understand it, from what I've read, the REAL problem with small powder charges in large volume cases is secondary detonation - which can explode the firearm. I'm not sure I agree with the explanations I've read, but then again, I'm not interested in being the crash test dummy for the secondary detonation theory.
My guess is that you shelled out big for a better .223 suppressor and expected it to be quieter. Or your wife/friends expected that. Am I close ?
I know I was disappointed with my M4-2K....and so I ended up getting a Trident9 and loading subsonic rounds, and also getting a 22 Mite with subsonic loads.
They ARE Hollyoowd quiet.