Muzzle brakes and flash suppressors deflect the sound rearward, so even though the decibel level is the same, more of the blast and noise comes back at the shooter. What's even worse, is the guy next to you on the range is shooting a large calibre rifle with a muzzle brake, that really blasts the people on the sides.
That is true of brakes, but is not generally true of flash suppressors. Good flash suppressors tend to direct the blast downrange away from the shooter, resulting in less blast from the shooter's perspective, not more.
It's certainly subjective, but IMO a 16" .223 with a Vortex FS is more pleasant to shoot than a 16" .223 with a bare target muzzle.
Excellent information. However the one thing not pointed out was the frequency of the sound wave. If you listen to a 357, while the DB level is loud the sound wave will have a wider wave form and not be precieved as loud as for example a 223 round with a shorter wave form.
Think of the ocean= a nice smooth rolling waves = 357
Think of the ocean = sharp white tips waves coming in very fast = 223
The dBA scale corrects for the frequency response of the human ear, and a .357 revolver is considerably louder in dBA than a 16" to 18" unbraked .223.
A 16" .223 at the muzzle is probably venting at 3,000 psi or less; a .357 at the barrel-cylinder gap is venting at more than 30,000 psi, and may actually be higher pressure at the muzzle as well (depending on barrel length).
a shotgun, running at low pressure with a fairly long barrel will be on par or quieter than most pistols, but not anywhere near a rifle of equivalent barrel length.
Measured dBA put defensive-length shotguns (18" to 22") very close to 9mm/.40/.45 pistols and .223/7.62x39mm/.30-30 rifles, though adding longer barrels to any of the above will reduce their peak loudness some.
You are right that a shotgun runs at a lower chamber pressure than a rifle, but remember that a .729" 12-gauge bore has more than ten times the cross-sectional area of a .224" AR-15 bore; in terms of sound production, that big increase in gas volume more than makes up for the reduced pressure at muzzle vent. The result is that sound levels are pretty comparable among most defensive shotguns, carbines, and pistols.