If the ammo originally fit into the gun and was fired, it means that the gun is probably fine and the brass is fine. Even if you don' t full length resize the brass should fit back into the gun!!! The bolt might close snugly....but it should close.

As you know, resizing (as well as firing) lengthens the brass...causes it to grow. I now believe your die is too long in the foreward end of the chamber allowing the brass to lengthen and then not fit back in the chamber.....However why one brand of brass doesn' t respond that way.....and my explanation don' t explain that.

As others have suggested....borrow another .300 Win Mag from someone and try it....I think you have a bad die!!