In fact, you're both wrong. Dud rounds, or bullets with a head but no powder are put into the drum of a revolver to get a close-up camera shot of the gun so it looks like it`s loaded with real bullets. The prop-master/armorer then unloads the weapon and puts blanks in it for the firing shot.
In the case of Brandon`s accidental shooting and death, the dummy rounds were removed but the head of one of those dud bullets remained lodged in the chamber. The armorer was pressed for time and didn`t check the gun properly to make sure it was fully cleared. When the blanks were loaded for the next shot and subsequently fired, the blank and dummy head acted like a real bullet and was fired into Brandon`s abdomen.
At first, the crew didn`t realize what was going on, thinking Brandon was acting the part until it was too late. The armorer was thought negligent at first but it was eventually ruled an accident.