[Unclenick]

When you test it over a chronograph. No equivalent alternative comes to mind. Check the average velocity of your original load, then load the small primer load up to match it. Look for whichever combination gives the lowest standard deviation in velocity.

By reaching the same velocity with a larger charge over the smaller primer you are doing it with more total gas volume. So even though the average pressure (what determines velocity) is the same, the peak pressure will actually be slightly lower. If the peak were equally high, given the larger gas quantity, your velocity would then exceed that of the original load because the greater gas quantity means more acceleration takes place further down the tube.