Magnum primers can burn faster/hotter than a regular primer, but I suspect the cup is thin. I do a lot of shooing small rifle primers in 221 Fireball, 222, and 223. I prefer Remington 7 1/2 or CCI BR4 primers because I just do not get pierced primers with them. You mentioned they worked fine in your 222, most likely because the pressure levels in a 222 are not as high as a 204. I know there are lots of folks that swear by Federal primers so they must work OK, but I still prefer the Remington and CCI bench rest primers, I get good results and they do have a thicker cup. Good Luck and Good Shooting