|A cartridge, round, ammo, or Shell (for shotguns) is what most people call a bullet.
The main parts of the cartridge are the
The primer is the circle in the middle of the back of the bullet
When you pull the trigger a hammer or firing pin strikes the primer causing an ignition of the gunpowder loaded in the cartridge.
This ignition causes the gas to build up in the cartridge. Since the cartridge in the chamber, it can't go anywhere, but the bullet can.
So it separates from the cartridge and flies down the barrel. If the barrel is riffled then as the bullet travels down the barrel it starts to spin like a football. This will give it stability as it leaves the barrel and flies to its target.
The case or back of the bullet stays in the gun and must be ejected for a new round to get loaded. This happens in one of the following ways