If you read the loading manuals you will notice that magnum primers are recomended in certain calibers for use with certain powders. For example, a 44 mag useing H-110, a mag primer is recomended because H-110 is a slow burning pistol powder and the hotter burning mag primer is needed for complete and consistant ignition. Same thing applies to rifle loads, and this isn't limited to magnum calibers either. Example, a 270 Winchester and a slow burning powder like H-4831, H-4350 ect will need a mag primer for the same reason above. The loading manuals talk about this, the Speer manuals in particular.