eli5 Why does the shell of a bullet come out of a gun but can shoot people?
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The shell that pops out is called the casing. It just holds the bullet, propellant (“gunpowder”), and primer together in a relatively secure package.
When you fire the gun, the gun hits the primer. The primer is impact-sensitive; when it experiences a sharp impact, it detonates. This sets off the propellant, which burns extremely quickly, turning to very hot gas. This very hot gases raises the pressure inside the casing to an extremely high level. The sides and back of the casing are supported by the metal of the gun itself (the receiver); the only side that can get pushed is the back of the bullet, which is stuffed into the front of the casing. So the super hot gas pushes on the bullet, pushing it down the barrel and accelerating it to a very high speed. Once the bullet starts moving the gun starts recoiling; usually, the recoil is resisted by some springs so that the bullet has time to leave the barrel before the gun fully recoils. When it fully recoils, the now-empty casing is kicked out (by a piece called an ejector or extractor) so that a new casing+bullet can come in.
So the bullet and the shell case are two separate parts. They are operating as designed; the bullet goes down the barrel at super high speeds to hit/destroy things, while the casing just carries the bullet and propellant in a single package, then gets dumped out the side or top of the gun once its job is done.
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