All ball powders, regardless of burn rate, need magnum primers. This is because the speed of the powder is governed by the coating on ball powders, rather than the physical properties of the powder granules. Very simply, ball powders need more fire to begin combustion than extruded powders.

Alliant 2400 is not a ball powder, despite it's relatively small granules. It lights pretty easily, hence no need for the extra fire of the magnum primer.