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.
