the 2400 will clean up when you get to 19grs.
I needed a magnum primer at 18 or below but it wasn't compatible to good groups.
many of us shoot 19.3grs of 2400 under a 250-255 plain base.
I generally shoot 8.5 or 9.3 grs of unique it just seems to work as intended without any drama, and it saves the 2400 for rifle loading.