What causes a bullet to shoot?

Answer 1

First, a couple of terms: What most people call a 'bullet' when referring to the thing you load into the gun, is more properly called the 'cartridge'. When the gun is fired, the projectile that comes out of the barrel is the actual bullet. A typical cartridge consists of a casing, a primer, propellant, and the bullet. The propellant used to be black gunpowder, but modern cartidges use a nitrocellulose-based* 'smokeless propellant'. We still tend to call the propellant the 'powder' though. When you pull the trigger on the gun, the gun's hammer strikes the percussive primer pressed into the end of the cartridge. This causes the primer to detonate, and the primer ignites the propellant (powder) in the cartridge. The propellant burns very fast, being completely consumed in a tiny fraction of a second, and this produces hot gasses. The gasses expand, creating tremendous pressure, and this pressure forces the bullet out of the cartridge and down the gun's barrel. Kind of like when you shoot a spitwad out of a straw.

*So-called single base powders are mainly nitrocellulose. There are powders called double-base powders that are mainly nitrocellulose but have 10-40% added nitroglycerine to give them more energy.