Connerm's advice is golden.

As a general rule, use the loads exactly as recommended until you've been reloading and shooting enough to experiment.

IIRC, the magnum primers produce more flame to get more dispersion of the flame for ignition so that burning bits of powder don't fly out of the muzzle, which is both a waste and a fire hazard. But I haven't reloaded in almost 35 years.

A friend & fellow reloader used to make his own sabots for his .30/06 to shoot a sub .22 cal bullet. He used light loaded pistol powder and rifle primers. Pistol powder burns very fast and with a rifle primer he got very fast ignition. Because he used a very light load he didn't build excessive pressure nor did he develop great muzzle velocity. He fiddled with the powder load (very minutely) to get good accuracy to 50 yds or so. He used the combination for squirrel hunting. His theory was that he would become a better marksman for deer if he learned how to shoot squirrels with his .30/06, but he didn't want to obliterate them. I think it's illegal now as I do not believe it is legal in my state to hunt squirrels with a centerfire rifle.