Why are they hazardous?

There is a risk of serious injury or death from any projectile propelled from the barrel of a firearm. An explosion of ammunition or the ignition of a propellant such as gunpowder may cause burns or penetrating injuries. Unloaded firearms carry a risk of causing injuries from mishandling, e.g. trapped or crushed fingers from a spring mechanism.

Some firearms have asbestos containing materials to protect from high temperatures during use, such as the barrel of a machine gun. The gun sights of some 20th century firearms contain radioactive substances for illumination.