Standard primer, versus Magnum primers?

Greetings folks!During a recent reloading session, I ran short of standard primers for my receipe of 38 handloads. Having noticed I still had a nice supply of magnum primers from my .357 mag days. Been always under the impression that in 38 loads mag primers would gift you with larger groups. Soo, I loaded my 550 up with mag primers, 4grs of red dot, and my Lee 158gr RNFP bullet design with 18/20 to 1 tin.Not wanting too many of these rounds around; due to the expected failures, loaded 40 rounds up and headed to the range. With my standard loads i had nice groups around 1.25 to 1.50” at 25 yards off a bench. S0 not expecting to much shot my first group at 1.125. well that was a fluke, so tried my next 5 shots and they shot about .940. So what the heck was going on here? so went home and returned 2 days later with my S&W M60-15. Alwars ran about 2.750 at 25 yards. Now with mag primers that group also schrank. Now this turned out just reverse from my expectations. both types were the CCI brand. Both started with clean barrels, one fouling shot with groups to follow. So what is the norm for the mag primers?? Any ideas would be appreciated??Have Fun! Rich22