in real life you can get a supressor for most calibers, even shotguns, but you dont call them silencers becuase no "silencer" can completly "silence" a gun even if the BANG noise is completly silenced you still hear the gun parts move, so gun sence says yes there is a silencer for the shotgun
Ah, you can get practical 9mm and .45 ammo in subsonic loads so no sonic boom, then set the pistol up to lock the slide (effectively single shot) so you wont even hear the slide slam back. Pretty quiet.
You could probably get a silencer on a rifled barrel shotgun, then load some sub-sonic slug shells, and pump so no slide noise just the striker hitting the primer. If you had a work bench and reloading setup available in game, and the approriate gunsmithing skill.
Oh, wait, all three of those are available in FO:NV...