Much of the earlier production brass by many of the manufacturers takes Large Pistol, standard and magnum primers. The first Winchester and Hornady brass I started out with took LP but I found with top end loads using H110 I'd get the odd shot where a noticeable amount of the powder remained unburnt. Then, Starline brass was the only manufacturer that used LR primers and I imagine that was to get a more dependable ignition with some of the slower burning powders. I switched to Starline, no more problem. The Winchester and Hornady brass that took LP primers I set aside for lighter cast bullet loads using powders like Unique. I 'believe' Hornady also changed over to LR primers and stamped these new cases with an R to signify that.