Hollow point bullets are used because they penetrate targets but also expand, which means the round stays inside soft tissues. The reason hollow point bullets expand and other types of ammunition don't has to do with the materials used and what happens to a bullet when it's shot out of a barrel into tissue.

What is the advantage of hollow point bullets?

Hollow points are an excellent option for home self-defense because they are less likely to pass through the target or walls and put family in danger. Hollow point bullets also mean that fewer rounds may be required to bring down an assailant, and that means better outcomes for all involved.

Why are hollow point bullets bad?

Virtually any hollow-point bullet will expand properly when fired into plain gelatin—it's the barriers that cause them to fail. Fabric, wallboard, and wood can “clog up” the hollow-point cavity and prevent expansion. Auto glass and steel can blunt the bullet and cause a similar result.