Oh, I've also used magnum primers in .243 win for the reason you mentioned: didn't have any normal ones. My .243 also happens to be a savage 99. I don't know if magnum primers have harder cups but I do know that my particular 99 has zero issues making them go bang, in all weather conditions.
Also, kudos on still using a SWEET old school rifle!!! A 99 in the original .300 savage? That says classy!!
I like my 99, it needs brothers. Ideally before I die I come to possess three more: a .250-3000 savage, a .300 savage, and a .358 Winchester are on my "I need it" list. Such a fine rifle and important piece of history in firearms development. I know the browning BLR and Winchester 88 are supposed to be stronger and more accurate actions…haven't seen anything deficient about the 99 and those others don't do anything for me, fancier or not. I love the appearance, ergonomics, and fast smooth lever of the 99, I'll take it over the blr or 88 every time as a compliment to either the win 94 or marlin 336.