I once did some experminting with different brands of primers. I was totally confused with what I found. Useing the same load and only different primers I found that some times with some powders I would get exactly the same velocity with standard and mag primers useing several different brands. I think I was useing 8 different primers. With other powders the primers had a very large difference in velocity. What totally baffeled me was that with 1 powder I started showing persure signs with standard primers long before I saw presure signs with mag. primers.
The only thing I am sure of is that you can never be sure what will happen when you change primers.