Small powder charge take up a small percentage of the volume of the case, and do not "be in the same place" or arrangement for each shot. Ignition is somewhat different from shot to shot, so group sizes are problematical. And this is why a filler is used, including stuff like oatmeal. Makes for a mess in front of the shooting bench.
Guessing, some 10 or 12 grains of 2400 would give less than subsonic velocities. I've used 20 grains with 169-gr. gaschecks in an '06 to get around 1,800 fps...
Anyhow, try using some sort of filler so that whatever powder charge you're using will be held uniformly against the primer.