What will kill you when you're shot with a gun?

Answer 1

The Bullet.

For a more serious answer gunshots can cause death in a variety of ways. Guns are not death rays and in most cases people who are shot do not immediately fall over dead.

Quick death generally only occurs with penetration of the brain, or destruction of the spinal column. If the bullet does not strike either of these structures death can occur as a result of organ trauma or blood loss.

Wounds to the heart will generally be fatal in seconds or minutes, wounds to the lungs will generally result in death when they leave the subject unable to breathe. The latter process can require several minutes, depending on the person. Trauma to other organs can also result in death if the injury isn't or cannot be treated.

Blood loss is the most common cause of death resulting from gun shot wounds. The wounded individual bleeds, eventually falls unconscious, and finally the brain dies due to lack of sufficient quantities of oxygenated blood.

Answer 2

Shutting down the brain. Either injury to the brain itself, or losing enough blood that the brain is no longer supported.