So I have a new Lee Deluxe Die set for 243 win and start neck sizing some unfired brass in the new collet neck die. All going well until on about the tenth case I hear a bang. The mandrel has pushed the aluminium top up and stripped the threads that hold it in the die body... I was applying some force as the instructions say at least 25lb but I am not a gorrilla and was really surprised. I know this cannot be normal and intend to take the die back for replacement but can anyone suggest what I might have been doing wrong?