Re: priming the 30-378
You will need to open up the throat of the tool with either a mill, dremel, or like piece to accept the larger cases. The shell holder is not the problem, the holder is.
I think I have one laying around that I opened up a few years ago. I broke the ear off the piece where your thumb pushes against to seat the primer and I never replaced it. I use my bench primer for everything now. Let me know if you want my old one and I'll send it your way..
"Character is who you are when nobody is watching"