Almost always I have seen an increase using mag primers. But I have sometimes seen a accuracy increase using std primers even in guns like the 300Win mag or RUM.

I tried some mag primers in my 308 once cause I ran out and accuracy was horrible but velocity up.

Generally, you want mag primers for powders slower than 4831 and regular for things below that IMO. I have never had good luck shooting mag primers for powders like 4064 or varget or even 4350