A modern cartridge has these parts: 1. the bullet; 2. the case, which holds all parts together; 3. the gunpowder; 4. the rim, part of the casing used for loading; 5. the percussion cap or primer, which explodes to light the gunpowder.
Bullets are small metal objects fired from guns. Many kinds of bullets are made from lead covered with copper. They are put into a package called a cartridge, which is put into a gun. The bullet is at the front of the cartridge (number 1 in the picture). The bullet is pushed out of the gun by hot gases at a high pressure. A powder called gunpowder (number 3 in the picture) is put inside the case (number 2). A small explosion from the primer (number 5 in the picture) lights the gunpowder. The gunpowder burns very fast. It makes hot gases. The hot gases are at a high pressure. The high pressure pushes the bullet out of the gun barrel at a high speed.