2 in 1= Adam/Michael sharing the same body. It's most likely Michael coming back. They been dropping vague hints ever since the 200th episode. But why the hell would he need a gun?
Maybe with Lucifer gaining control of whatever spell Crowley put on him he lost his original control over the cage. Or something like that and it allows Michael/Adam to break free.
Another theory is Cass took Kelly and her baby and hid them in the past. The Nephilim, now fully-grown in the present, comes to save Cass. Still though why would the son of Lucifer need a gun??