The meaning of an accessory projectile

A previously expended pistol casing was removed from a gunshot wound after an apparent accidental shooting. Examination of this foreign body indicated it had ridden the bore ahead of the bullet. Further investigation disclosed this as a deliberate but ignorant means to load an already loaded weapon. The assailants then fired at a passerby and killed him. Disclosure of the foreign body by X-ray, recovery and examination at autopsy were used to corroborate the circumstances and allowed charges of criminally negligent homicide to be placed.