I've posted on this on my Sherman Wildcat page because of all the sizing problems people have.
The problem with die makers is, they tend run on the loose side because to properly size a magnum case, you are only about a thou short of cracking the die. Another problem is they are made for saami spec and for throw away brass, or at least brass that can only be fired 3-4 times. Most dies will size once or twice fired brass ok and then problems start because the brass doesnt spring back as far after a few firings.
When a die maker gives you a spec of .530", for example, it doesnt mean it will size to .530" because of spring back.
Ive dealt with 3 different die makers for my stuff and here is what Ive learned.
For a magnum size case head, you need to ask for .0045" under chamber size to get .0015" -.002" sizing on the web, which is what you need. You MIGHT get by with .004" under but with heat treat and polishing differences, you are taking a chance!