Are you a re-loader at all Fox84? The reason I ask is because then you might have the tools and knowledge needed to determine how much the PMC fired case is stretching.
It's possible that your PMC cartridges are on the short side of specification, while your chamber is on the long side of specification... not necessarily excessive headspace... just a combination of extremes. These conditions could, I say could, result in a case head separation.
Exceedingly stretched cases, indicating "incipient case head separation" would be a clue.
Worst case scenario is that your new gun chamber, barrel extension or incorrectly machined bolt is creating a out of spec headspace condition. A gunsmith can easily check this for you, or you can return the gun to Stag for inspection.
Shoulder Drive Nicholson Club