What happened was a gun was loaded with prop bullets. No gun powder in the casing, already detonated primer and the bullet. Looks exactly like a real bullet, just no way for it to actually fire. One of the prop bullet's primers wasn't detonated. When the gun was fired during a previous scene, the primer had enough force to push the bullet into the barrel of the gun. Later on, the gun was used again, this time with blanks. IIRC it was a full load blank. When the blank was fired, it shot the bullet that was already in the barrel and hit Brandon Lee. Neglegence killed him. No one checked the gun after the first time it was used and it led to his death. There was a documentary type show that had Linda Lee in it talking about Brandon and his fathers death a few years ago.