So that just happened...
Now Walt's out of town for what, at least 8-9 months*? What draws him back to ABQ?
My guess is that he finds out Jesse is cooking for Jack's gang, so he comes back to town to kill them all. I thought there was a chance he would let Jesse go after Hank died, but now it seems clear that he has 0 interest in letting Jesse live. And Jack & Co. crossed him by using Jesse instead of killing him, so they'll have to pay for that.
Jesse told them about the tape he made with Hank and Gomie, I think Marie would have known about that, so maybe Jack goes after the rest of Walt's family to tie up loose ends. That would be a different reason for coming back than my first thought, but either way I'm thinking the huge gun is to take out some neo-Nazis (and Jesse).
* 52nd birthday in the flash-forward in 501, 51st birthday in 504, the montage in 508 was ~3 months I think?