A firearm is a mechanical device that uses pressure from a burning powder or an explosive charge to force a projectile through and out of a metal tube; a weapon, especially a pistol or rifle, capable of firing a projectile and using an explosive charge as a propellant. A bullet is a lead or copper encased projectile fired through a rifle or handgun barrel. 2  A slug is a lead projectile fired through a shotgun barrel. 3  Shot is a group of lead, steel, tungsten, or bismuth pellets fired through a shotgun barrel.