I live in a townhouse. There's no way I'll ever fire an AR inside. Besides what it does to the bad guy, the muzzle blast would destroy my hearing and my family's hearing forever. The tactical operators who think using an AR for home defense is a good idea should try firing one indoors sometime without hearing protection on.
In terms of peak dB, an 18" barreled shotgun, a 16" to 18" barreled .223, and a 4" barreled 9mm or .45 are very close (around 155dB). Rifles and shotguns may have a deeper sound, but the peak dB is similar.
That's because peak dB is not only correlated with caliber/powder volume, but with barrel length, which determines the pressure gradient at the muzzle. Shorter barrels make any caliber much louder, as do muzzle brakes.
The loudest of all defensive firearms, per tests I've seen, is the .357 magnum revolver, due to the barrel-cylinder gap, 4" to 6" barrel, large gas volume, and high working pressure.
My shotgun is loaded and sitting in the bedroom closet but that's a weapon of last resort. I keep a 9mm in the nightstand. With an 18 round magazine, that's all I really need. I actually fired a 9mm inside my house one time and the sound of it in a small room was incredibly loud.
It would be incredibly loud, indeed. But a 12-gauge or a 16" to 18" .223 (unbraked) would not be much worse.
Ha anybody done an actual practical test pertinent to this issue? Should be pretty simple if one has enough money to build two three walls, spaced at least 9 feet apart and build of various typical materials. Use typical HD shells and cartridges and at various spacings. If it has already been mentioned I apologize for my redundancy.
Yes.
http://how-i-did-it.org/drywall/results.html
55gr .223 SP and JHP tended to penetrate fewer walls than 00 buckshot. In that situation (townhome) I'd personally go no heavier than #4 buck, and possibly lighter.
If I had neighbors right on the other side of a wall, I might even consider .223 40gr JHP, even though it penetrates abysmally in gelatin (6" to 8"), and take it as a given that multiple shots would be required. Federal 40gr "Blitz" JHP was a reasonably common SWAT load back in the day and was apparently rather successful, while keeping wall penetration to a minimum. My own AR is loaded with 55gr JHP, though.