Ken Waters lists max loads of IMR4198 at 21gr, whether with the 6 1/2 or 7 1/2 primers, so I doubt if they were "magnum" primers. Thicker cups, yes. In my .222, 21gr flattens the 6 1/2s, so I use 20.5gr with them. Accuracy is the same with either primer or powder charge, right around 1/2" at 100yds with 50gr Blitzkings.
