Rule of thumb over the year is to use Magnum with ball powder and Standard primers with with stick powders. Ball powder burn rates are controled by threir coatings and can be harder to ignite. Very large capacity Magnums like the 378 Weatherby have been suggested to use Magnum primers due to the large volume of powder. Very cold weather -0 can cause slow ignition and magnum primers work well then. Do not substitute a magnum primer without starting low and re-developing you load.