As driven by the FBI’s handgun ammunition requirements, bullets that expand wide, hold together and penetrate deep—even when passing through intermediate barriers—is the current trend. This is because law enforcement officers often have to shoot it out with bad guys who are smart enough to hide behind things like car doors and walls. This ammo is also relevant for personal protection, because bad guys don’t go around naked and some conventional bullets will fail to expand after passing through thick leather jackets or similar clothing.
The problem with using these robustly constructed bullets in your home is that they can, theoretically, pass through a bad guy with enough remaining velocity to still injure a family member. They could even continue out into the street or into the next apartment. Realistically, this is not that likely; once an expanding bullet has utilized enough of its energy to expand and pass through a bad guy it has limited remaining velocity and is unlikely to pass through any wall used in residential construction.
But, what if you miss a home invader with one of these FBI-approved projectiles? In that case, it is very likely to pass through a wall and continue on, maybe into another room, home or the playground next door. This could all be very important depending on where you live and whom you live with.