X2 on the B's comment. I have used mag primers on 303 british, but worked up my load with those primers from the start. In some loading books I have seen other loads with the note to use mag primers for certain powders. I never tried mag primers with the BL(c)2 in my 7.62x54R as I was working with other powders at the same time and found H4350 was just way better and so have stuck with that.