Since the hypothetical is limited to handguns in front of me at home, and since presumably I wouldn't *know* that I'd be limited to one shot at the time I heard the scream, and since precision is required being that the loved one is adjacent to the BG, this is a job for the CZ75; close second is the 5" Raging Bull in .454 casull. Since one may very well not have a shot at chest/COM, since it could be occluded by the loved one, or could pose a danger to the loved one by shooting through the BG's center of mass, I'd likely go for a head shot (low rear) or neck shot.