The Speer #13 manual frequently recommends magnum primers for ball powders (at least for some of the calibers I reload for). I tried the same load in .223 Remington, one with CCI 400 primers (small rifle) and the other batch with the recommended CCI 450 primers (small rifle magnum). I couldn't see any difference in accuracy. I don't have a chronograph. This was during warm weather. It might make more of a difference in very cold weather.