Your 16 fps doesn’t seem that unreasonable although I would expect the opposite considering once the case is formed the capacity is slightly increased.
Are these the same lot number brass?
How does the new brass to old brass neck tension compare ?
that's why i don't think it's case capacity. not just same lots, the exact same brass. when i take a piece of new brass and then load it a second time, it speeds up.
i really want to get one of those fancy presses that measure loading force, as i have no way today to measure neck tension other than measuring the diameter of the brass, but that doesn't account for the annealing/hardness of the brass. it's consistent when it put it through my AMP annealer every other round but different when it's new.