Shotguns require too much user input. It is too easy to mess up a shotgun under high stress. And any malfunction with one is a fight stopper.

If you are worried about noise, buy a suppressor. While out won't turn an AR.into indoor hearing safe it will moderate the noise. A 12 guage indoors isn't fun.

As a defence gun the 15-22 is questionable at best. While it can work the margins of error are not in its favour. It will be an investment equal to the 15-22 but a suppressed short barrel 5.56mm rifle loaded with frangible rounds is probably your best bet. Simple user interface, able to easily take a white light for target ID and a red dot optic. easy too use, easy to fix if it stops rapidly able to reload if you need to.

Of course getting one now may be difficult our expensive.

Forget about 50% of the garbage in this thread about not killing the bad guy. If you shoot someone you are employing lethal force. By the very act you are, by law, trying to kill them. Using anything that reduces this level of force is silly. If you aren't justified in killing them with your first shot you shouldn't be shooting at all. Any decent lawyer will be able to counter any problem questions about hollow point bullets or equipment by calling in experts. Remember it all hinges on you being correct in shooting in the first pace.

Get a rifle, get proper training with it. In the mean time if the 15-22 is all you have, so be it. It isn't a combat weapon, but it is dangerous.