The ones which are of a hollow canister shape, and emulate the look of silencers, deaden muzzle sound 'some'.
Actually, I was googling to better describe what I was talking about, and it appears that fairly recently the ATF made them change the design of canister-look flash suppressors and fake silencers, so that they have a sleeve down the bore on the inside, and do not open up to a large diameter inside. This change apparently because the ATF felt that the open-hollow sort deadened muzzle sound too much for their tastes.
As far as I know, that doesn't reflect legally on older models. just a 'from now on, put sleeves in those' ruling.
At any rate, the type i was referring to are apparently no longer legal to manufacture new. Explicitly because they deaden sound a bit too much for ATF preference.