Originally Posted by Old_Crow
You have a firewall between apartments. Shoot 4 layers of 5/8 drywall with any 9mm HP round. Have a pile of dirt behind the drywall so you can find the bullets. You will see there isn't much left of the bullet laying on the dirt. Very few bullets will even dig into the dirt.
Even so, which is not always the case (even seen the "student slums" near most large campuses?), "it would be bad" for the round to penetrate your wife's / child's / roommate's interior wall with remaining lethal energy... or the glass of a window facing your neighbor's loved ones' space ... or windows facing the street and hitting a passing car or pedestrian.
This is where a frangible slug is the better choice, IMO.
And while marksmanship is likely to be the best policy for not over-penetrating, the Bad Guy or critter may not act in a predictable way, might get the drop on you, might cause an AD during a struggle and launch a lethal projectile in an undesirable direction.
No battle plan survives first contact with the enemy.