I am going to step out on a limb an say that the belted magnums seem to use the same Chamber headspace gauge. You don't heaspace the barrel off of the shoulder as you would a non belted case.

If you are a reloader this is why neck sizing a BM is important for good accurate loads. If you do full length size you want to use a Case headspace gauge to set up your die and only bump the shoulder back .001"-.002" on a fired case.