the supersonic noise of a 300 blk suppressed vs a 223 is similar - but it also depends on the powder used.
the 300 blk, because of its case volume rely's on what are typicall magnum pistol powders. because of how they burn they're not always the quietest powders to suppress. you'd have to fool around a bit to balance both sound and function.
bear in mind that the cycling action of an ar-15 is somewhere close to the 100 dB range itself.
i'd say both my 223 (20") and my 300 blk (16" carbine gas rifle and 9" pistol) are both about like shooting a 22lr unsuppressed - give or take. I can shoot them without hearing protection, but prefer not to as i've already done enough damage to my ears over the years from doing silly things before suppressors were legal here in michigan. however - with the suppressor on - even basic foamies are more than sufficient to make for a pleasant day of shooting.
