Looks like several factors, any one of which or in combination, could pose a problem...
1-new barrel.
2-brass was sized for the old barrel and possibly short in headspace.
3-max load listed as such but "OK" in the old barrel.
4-changed the primer.
In effect you used a load that featured brass that had been sized for the chamber of the old barrel, is listed as maximum in a manual, AND modified it with a different primer AND fired it in a new barrel. Looks like you got lucky with just a partial head separation.
