Ash as others have said it sounds like a one off. The vertical split-as I call those that run lenght ways- is normaly due to brittle brass or faulty in that it is thinner there then it should be. As it is new it may well of not been annealed at the factory properly.

If you have this happen with any more from that batch take the lot back to where you bought it and have them send it to Winchester. I would be looking for a refund if that was the case.