My handloads with H4350 (and I use tons of it) use standard large rifle primers. Federal and Winchester primers both work well. H4350 is relatively fast as powders for large cases go, but the 375 H&H is on the small side of large (compare max loads to 378 Wby and you'll see what I mean). My handloads with Hornady 300 gr softpoints are amazingly accurate with top notch velocity, using standard primers and H4350 (in a Ruger #1). If you load powders slower than H4350, then magnum primers may be needed, but H4350 responds well to standard primers.