All my R - P gets sized to 40-65 for my Browning BPCR, as it is slighty smaller at the base. FC (heavier) cases are used in my Browning SRC, in new-fangled fad powders. The W - W (lighter) is used for HB in my Pedersoli, Gemmer custom, 45 - 70.
I read in a Rifleman article once, that one grain of smokeless has the same volume as 8 grains of brass. I don't know if Black is the same? The heavier the brass the higher the pressure, if the powder charge is kept constant. That is why Joss is right; - DON'T MIX BRASS, especially with top loads.