True, as one who has "been there", I will tell you that your efforts will likely not be as fruitful or satisfying, suppressing .223 Rem.
Some claim to have sub-sonic .223 ammo, and that it successfully operates gas ejection mechanisms, some say bolt action recommended. I have not personally seen acceptable .223 ammo which leaves the muzzle at 1,000 fps.
It is far more rewarding, and challenging to boot, to load 9mm with very heavy bullets, as high as 180 gr., and shoot them out of an AR-15 made for that caliber, having removed the standard flash-hider, get the drift?
You can do this by legally transferring a suppressor such as Sionics, designed originally for MAC-10, to your name, and then become a happy camper.