I have a dB meter app on my phone. I have no clue how accurate it is, but it's interesting to use. I've been at my kids' school functions where the loud music can hit well over 100dB. And naturally I've wondered what kind of reading it would give for gunshots, so... I brought some Blazer .357 and some reloads with just a primer in them, to compare. I was pretty surprised at the readings, though I know dB readings are not always as cut and dried as a raw number might imply.
Sitting here at my desk, I am able to clap my hands a foot or so away from the microphone, and it gets up to 104.7dB. At the range, the mic was probably 4 feet from the sound source, and with the .357 magnums, it read 120.2dB. The small pistol primer all by itself, a sort of .38 Colibri round without any projectile, read 109dB. With earmuffs on, that 109dB sounded like almost nothing, so the number was surprisingly high. A difference of 11dB is more than it seems, since it's a logarithmic scale. But also, the full blast of .357 is a whole bundle of frequencies letting off with a blam! sound, compared to the polite little pop of a primer. Kind of like a needle poking you vs. a sledge hammer.
Anyway, it means nothing, proves nothing, and I wouldn't fire a primer-only cartridge indoors or the back yard here in the suburbs. I'm not sure what burned primer gasses and chemicals get belched out the muzzle, gooking up the cylinder gap, and 109dB is too loud for inside, though I doubt the neighbors would think to call the cops. So there we are.