The magnum primers just keeps the powder fouling down and makes for more consistant ignition with ball type powders, which can be difficult to ignite and burn consistantly, or so the rumour goes. I've tried some of the ball powders mentioned, in other cartidges, with both standard and magnum primers and don't find much difference, other than a very negligeble velocity increase and unless it's in a rifle accurate enough to make a noticeable difference, like a Hunter Bench Rest or it's ilk. bearhunter