Raimey,
I don't use Bertram cases, so I can't speak from firsthand experience, but over the years have seen others complain about Bertram cases, some that the cases were brittle. I think Bertram provided the cases and someone else loaded the ammo. The use of .266" bullets is also a concern, but if justified may be a blessing in the long run. You should "slug" the barrel to find out the actual groove diameter. The cartridge was intended to use .260-1" bullets, like the 6.5x48R and 6.5x58R, but if the maker did use the more modern .264" groove diameter barrel, you would be freed from the requirement to resize jacketed bullets. Bertram cases are very expensive and to save as many as possible, I would pull the bullets, empty the powder and deprime. Then I would anneal the cases (taking note of where the previous owner experienced splits) and fireform them with a filler and no bullet. This method of fireforming is much easier on the cases than firing a full power bulleted load to fit the cases to the chamber. Hint: try one or two first to test the procedure before pulling all the bullets. If the annealing and fireforming is successful, load some with bullets and a mid-level load and try them. Neck size the cases when loading these, to provide the cases with as much support from the chamber walls as possible. Good luck, I hope it works out.
Mike
