Gentlemen:
I have always used magnum primers when loading ball powders W760, Ball c lot#2 H335 etc. for 60 years and the use of magnum primers on all ball powders was standard practice in those days. I know that powders have improved over the decades, so I went strictly by the book when loading my 7X57 with StaBall 6.5 with standard LR primers. My ES and SDs are very high on different charges. Groups go from outstanding with very low ES and SDs too horrible in the same sessions with high ES and SDs. I think that it is inconsistent ignition problem and should use magnum primers. I'm loading a 7X57. What say you?

Paul K

My favourite .375 H&H load with a 300 gn bullet is Staball 6.5 74.50 gns, ignited with Federal 215 Magnum primers. I am achieveing MV a little higher than what data suggests for that charge. The load is very consistent. Case fill would be around 95% I guess.
Obviously not the same as what you are comtemplating but if it were me I wouldn't hesitate trying magnum primers for your load development. I have at times substituted magnum primers in already developed loads in various calibres to see what differences occur. Results have varied and mostly shown slight velocity increases. With magnum primers I suggest starting a grain or two lower than where your present load is. Better powder burn is also helped by charges that completely or almost completely fill the available case capacity if that's possible.

Could be a number of different things causing your high SD, but I wouldn't hesitate to try the magnum primers. I do just the opposite and use standard primers with W-760 in my 7X57. Staball is a little different animal though, it might benefit more from the higher energy magnum primer. I haven't got around to trying it in mine yet but I intend to.