Some years ago a couple of guys in the neighbor town was caught for illegal hunting. They used a 223 with a 30cal can, later when their equipment was for sale at a lokal gunshop I bought the can. (In Norway we don't need to apply for a NFAform or like it, it's a free item to purchase) Our weapons have to be registred, but cans are free.
Anyway I know these guys loaded their own ammo to get it subsonic and they was doing it for years before they got caught.
So to silence a gun it is important to have the right ammo.