It seems a number of set and cast members were out the back plinking at tin cans earlier in the day, maybe they got called off their 'range' and someone left one live round in the chamber, put the gun down and then someone else picked it up and filled the gun with five dummy rounds or they simply did not clear the chambers of live rounds before bringing it on to the set, either way someone put a live round in the gun and as you say someone did not check before handing it over, I would suggest its way wrong to have live rounds on a film set to play with out the back, has to be an accident waiting to happen and someone must be held responsible.