I have 100 once fired 300 Winchester Magnum cases that I would like to use to make 308 Norma Magnum cases. I have a set of 308 Norma Magnum RCBS loading dies and have heard that all I have to do is use the sizing die to move the 300 Win Mag shoulder back approximately 0.100". Then, trim to 308 Norma Magnum length. Will I have to fire form , and if so, what will a fire form load consist of?