Accuracy means different things to different shooters. Some think 3" patterns @ 7yds is fantastic. Others use nra x-ring standards as a gauge for accuracy. Others like myself use 6-shot groups (or whatever the revolver holds) when testing loads for accuracy with revolvers looking for x-ring accuracy.
What you've found is something that you should of been doing all along. Most casters don't do this kind of testing and they really should.
Mag primers don't have a huge affect on over all pressure of the revolver/pistol loads when fast burning powders are used. They will affect the short start pressure of the load. When you see large increases (20fps+) in velocity or better accuracy with a mag primer using bullseye/clays/titegoup/fast burning powders. That's telling you to look at your bullet fit or alloy's hardness/crimp.
Not all bullets are designed/created equal. Same powder charge but 1 bullet could be seated deep, bullet base/bottom drive band could be more efficient, etc. The bullets crimp groove depth/size/amount of crimp can affect how a mag vs standard primer affects accuracy.
Myself I use mag primers to my advantage and do a lot of testing with them. These are the things I test/look at: Crimp/alloy/bullet diameter
Too many times I see on this website (and others) "I only crimp enough to remove the flare." . I work up loads using standard primers and fast burning powders using 8/9bhn alloy/cast bullets. Then I switch to mag primers and retest. If there's an increase in velocity I increase the crimp. If the increase in velocty with the mag primers is still there after increasing the crimp I increase the diameter of the bullet.
Doing these standard VS mag primer tests with soft alloys and fast burning powders lets you dial in your bullet diameter, amount of crimp & alloy issues. At the end of the day I you see no or minimal changes in accuracy or velocity when switching between standard vs mag primers you have everything dialed in.
These type of things tend to show up more with low pressure loads (16,000psi or less). But when you dial everything in with low pressure loads it makes it easier to find why loads fail at higher pressures. Using mixed cases should have more of an affect on your loads/accuracy then a standard VS mag primer with pistol/revolver loads out to 50yds.
Anyway, it's a good thing to cross check a load/cast bullet/alloy with mag vs standard primers using fast burning powders. Any changes are telling you something isn't right. I'd be taking a look at your alloy/bullet bhn 1st. Bullet hardness is sooooooooooooo over rated!!!! I've showed these targets before, they're not hand cherry picked by any means. Nothing more than the targets used that day testing loads.
8/9bhn alloy cast bullets sized correctly & the reloading dies setup to maximize the case neck tension and crimp using mixed brass.
18,000psi+ clays/45acp loads 3 different bullets 5-shot groups @ 50ft all 1" or less 4.3gr clays
Playing around sighting in a contender @ 100yds 1 1/2" 3-shot group (bottom right) +/- 25,000psi load
The 6.5gr load of bullseye is 30,000psi+, 2 different bullets/2 different loads 6-shot groups @ 50yds 357mag all 2 /14" or less
5 different bullets/7 different powders loads up to 25,000psi 6-shot groups @ 25yds 13 targets 1 1/2" or less
Anyway it's a good thing to cross check you loads with mag vs standard primers. When you have little or no affect on your load it makes it sooooooooooo much easier to use the same alloy to dial other loads in. Especially when using low pressure loads.
