Two first hand observations.
A friend who is an SOT and avid experimenter built two revolver cans, both with .375 holes and baffles, each was of different capacity/volume. They were conventional screw ons and he spun off and threaded the 4" bull barrel on a S&W 65 for the experiments. We found that conventional subsonic .38 special was reasonably quiet with both what he considered the high volume and low volume cans, almost to the level of the 9mm subsonics in a conventional can. But the cylinder gap generated some noise. When fired with .357 rounds it was like the can was not even there almost...lots of blast and noise. No instruments involved to measure it, so that is a subjective observation.
He also picked up one of the 8mm Nagant's, threaded on a 3lug HK style muzzle device and attached an MP5 can. Quiet as a mouse. It was comparable to shooting a suppressed .22LR subsonic in an autopistol. That 8mm round is not very potent. The MP5 can sucked up all the gas.