I've had the same problem with cast bullets. The neck of the case is too small and part of the bullet gets caught on the edge of the case. Then, when you seat the bullet it pushes down that edge and ruins the case. What I've done is chamfer the case mouth, that seems to let the bullet ease into the case better. Just use a regular case chamfering tool and try not to take too much material off, just enough to let the bullet in.