It would be interesting to see a chamber cast, but in lieu of that, what was the o.d. of the necks and what size slug would fit in the necks of the first fired cases?

I have had fired cases that wouldn't fit the bullet that they fired, but that was due to the pressure of the load and the neck didn't fully expand.

Was the length of the case over on the Remington cases(not that you can tell at this point)? Could the case have pushed into the throat compounding the bore issue and spiking pressure?

All of that aside, beautiful rifle and I am also curious about your range report with the Hawk slugs.
Good shooting.

Tom