From what I remember...you will see a higher std. deviation if you were to check them with a chronograph (same load with magnum vs. std primers). The trend is generally a higher std. dev.= less accurate. So, if you’re trying to shoot the gun for groups, it may not be your best effort. Theory is inconsistent burn from a more turbulent ignition, burns from the front back....blah, blah, blah... I use Winchester large primers which are suitable for std & mag loads, and they are just fine.