Originally ALL small-rifle primers had thin cups, because all the cartridges that took them were relatively low-pressure, none developing pressures over 50,000 PSI. It wasn't until the 1960's, when the .223/5.56 appeared, that the first so-called "magnum" SR primers appeared, and then they were primarily heavier-cupped, not "hotter."
Over the last half-century more thicker-cupped SR appeared, mostly because of more factory cartridges based on the .223 case that operate at over 50,000 PSI. Some are hotter-burning and some are not.
The .300 BO's standard SAAMI maximum average pressure is 55,000 PSI, which aside from powder selection is probably why Hodgdon used a magnum primer.
The primers Jeff lists are all thicker-cupped.
Yep, Johnny Buffalo is spot-on, as usual. Absolutely totally correct.
Most handloaaders assume that magnum small rifle primers are hotter than standard small rifle primers. And that simply is not true. The only difference is the heavier cup ... so that they can withstand higher pressures.
Blessings,
Steve
