Using NFA registered items like short barreled rifles,sawed off shotguns,full auto,and suppressors are not recommended for SD shootings. It kinda falls under the lawyer hay day category like trigger jobs,modified safeties,and reloads. I only know of 2 SD shootings with FA so there is not much case info on it. Its not illegal and they say justified is justified but a lawyer could bad mouth you.
You could want to use one indoors for the same reason LE/SWAT sometimes do. So you don't blow your eardrums out in tight quarters with a loud firearm like a .45 or AR15. It doesn't make it all that quiet. Just tolerable.