Just for the record, I am a builder and I know what a modern fire wall is. Its simply drywall from the foundation all the way to the roof in most areas. There might be some jurisdictions that call for more, but most of the Western U.S. simply uses drywall. Some localities call for a Sound Wall and this is usually two 2x4 or 2x6 walls with drywall on each side, so that is 4 sheets of sheetrock. I now live in a duplex myself... ironic I am a builder and am renting... I know, it's a long story.
But in my case, one of the garages is between each unit which I think cuts down on the risk... especially since my garage is actually pretty full at this moment. But I am still leery of over penetration, so I am careful... I have a j frame 38 spcl as my main bedside handgun, but if given the chance I would rather use my Winchester Defender 1300 12 GA, which I have loaded in order 2 #5's then 2 #4 buckshot and the balance 00 buck. I figure if after 2 #5's and 2 #4's they are still attacking, then it's time for 00. Which are my reloads also.
So that is my .02 YMMV
Good luck and stay safe.
JohnnyOrygun