LP primers will not fit in SP or SR primer pockets.
What powder are you using? Are you crimping your cases? Magnum pistol powders generally require a firm crimp for reliable ignition. SR primers should be wholly adquate to light the fire.
If you are having misfires also check that you are getting a good dimple on the primer from the firing pin (iow, it may be the gun, not the ammo)
Is this out of a rifle? What type? I know handi rifles have had a problem with light strikes at times I think due to a transfer bar problem. Either way I would think the misfire would be a firearm problem not a load problem, assuming when you say misfire you mean click....nothing....
Try to answer all all the question I have stirred up. Its a thompson contender. Been just firing primers to see if its the gun or old primers. Here what I have been trying its sp in the mag and sr for the max. Thought dia of all primers were the same except for hardness Thanks to everyone for the advice Found two problems one broken hammer spring and hammer extension.