I am wondering if you have a tight bore or tight chamber. I have run into this before with a few of my rifles. The chamber is match, and the bore, instead of being .277 was actually .272. This caused blown primers and massive case swelling on Speer book upper end loads. Not max loads, but getting up there. The starting loads were pretty warm in this instance. Try neck turning and maybe try a chamber cast and see what that tells you. Only other thing that comes to mind is that the neck is being pushed into the lead and being crimped into the bullet, which will definately cause massive pressure problems. Hope that you get this problem solved, because an explosion 4" from your face is not a good thing.