I know belted magnums headspace off the belt on the first firing, and then the shoulder can be used for reloading. I have also read that SAAMI is usually larger than actual brass/belt distance, so extra headspace is inherent.

Is it advisable or contraindicated to headspace with a go gauge that will allow extra room for shoulder to blow out, or use a regular go gauge to set install new barrel?

I have been unable to find a 7mm rem mag go gauge btw.