Jessie,
What kind of standard primers where you using? I haven't used but a handful of CCI and none of them with 2400.

I have a theory as to why I got better results with magnum primers. My loads were less than full powder. I hadn't gotten to the place where 2400 was working optimally, generating enough pressure on it's own to "play nice".

Maybe not, just kinda thinking out loud here. What do you think?