if you’re fighting an assailant who is wearing a BPV, aim for the femur first. They will be unprotected and a load of buckshot to the femur at close range will put a man on the ground instantly and/ more than likely, result in rapid bleed out. I think 4 buck is the preferred buckshot incidentally especially indoors and especially if you want to shatter a femur and shred a femoral artery. I heard the military determined that 4 buck was the optimal shotgun round for military applications.
We had MB 500’s in Iraq and quite honestly I can’t think of a better CQB weapon. Usually just the sound of one being racked was enough. Of course wartime is different than protecting one’s own houselhold, but I just had my soldiers go straight for the neck. 00 buck shreds it like it was made of jello.