The_Dunedan

Hollow-point ammunition is exactly what it sounds like: the bullet has a hollow cavity molded into the nose. This causes it to expand violently when it hits something, which not only results in greater Kinetic-Energy Transfer ( a crucial factor for causing shock and disabling a target ), but causes the bullet to slow down very quickly, which helps prevent over-penetration.

HP ammo is superior for self-defense because it causes greater shock-trauma and bleeding to the target, is less likely to overpenetrate and injure an innocent/noncombatant, and imparts more physical force when it strikes. Conventional "ball" ammunition will simply blow through a target: this causes potentially-fatal injuries, but the injuries are not fatal as quickly, less shock is delivered, and the risks of hitting something behind the target are greatly increased.

Because HP ammo is designed to expand upon impact, it is terrible at penetrating body-armor. Ballistic body-armor works by spreading impact-related force out over a large area; to get through it you need something "sharp" which can more easily part the fibers. Shooting through ballistic armor with HP ammo is somewhat analagous to splitting logs with a baseball bat ( HP ammo ) as opposed to a maul ( Ball ammo ) or axe ( AP ammo ).