Always go by the manual. Moderate to quick powders normally do not work well with magnum primers. 800x is definitely a moderate speed powder. Chemical compositions, shape, and coatings of powders all contribute to whether a standard or magnum primer is most desirable. One example would be 2400, while a slow pistol powder, its composition makes a standard primer the most desirable. Always refer to the manual. Those companies have pressure testing equipment, we don't.