Originally Posted by Zombie Steve
I've heard about 2400 and magnum primers. I have one load with a 158 grain XTP I worked up before I started casting. I followed the recipe in Lyman 49 (including a magnum primer) and got almost the exact same results (also shooting a 4" barrel). Twice around the wheel and my extreme spread was 38 feet per second (13 standard deviation). Seemed pretty darned consistent to me. Cases fell right out.
Is there a general rule as to which powders require a magnum primer, and which do not? Is there an imaginary line somewhere on the burn rate chart that when crossed might require a magnum primer?