I think the problem is simply a slightly under-size chamber. I have a JP Sauer 30-06 that will chamber a factory round but no case resized after firing in another rifle. Eventually I found an ancient sizing die that was just enough smaller at the base. RCBS should have a small base die that would be worth trying. If not, I wouldn't hesitate to have a good barrel man run a proper reamer in and settle the matter once and for all. For a lark, you might try wrapping a layer of Scotch tape around the case at the tight spot and running it through you size die.

Bill Ferguson