When To Use Magnum Primers
Q: I've obtained five canisters of discontinued Winchester MRP Ball powder that I intend to use for loading my .240 Weatherby Magnum. Should I use a standard or magnum primer to ignite this dense Ball powder?
A: Most manual data recommends the use of magnum primers with belted magnum cartridges with large powder capacity. If a powder is difficult to ignite, it may require a Magnum primer. You can determine this by shooting a given load with and without a magnum primer. With some powders, particularly slow burning ball and spherical propellants, the use of a magnum primer can reduce standard deviation and extreme velocity spread by a considerable amount. When the powder is ignited completely and consistently (as indicated by extreme spread and standard deviation) accuracy will often be improved as well. Over time I've used such slow burning powders as Re-22, W-780, AR2213SC, WMR, WXR and Supreme 780 in my .240 Weatherby, but always with a Magnum primer.