The trigger of a semiautomatic pistol is pulled, and a firing pin in the back of the gun darts forward, slamming into the primer on the rear of the cartridge holding the bullet and causing a small explosion. This ignites the gunpowder in the cartridge, blasting the bullet out of the barrel with tens of thousands of pounds of pressure. That blast goes both ways, driving the left-behind shell casing backward hard enough to leave an impression from the inside of the gun on... (GunPolicy.org)