Johnny Ringo said:

Walt is probably buying that M60 to go up against Hank and Co.

But then I was like -

If he really has gone full Heisenberg then it's probably equally likely that he is going after former associates like Todd (aka Meth Damon) and Lydia. Erase anyone with links to Heisenberg.

Even as far as Walt has fallen, and as much as he has done to unintentionally hurt his family, I can't imagine him setting out to physically harm Hank.

My thoughts are the M60 is to tie up loose ends related to Todd and Lydia, and that the ricin might be his personal exit strategy.

But, you don't need an M60 to take out just a couple of people. Obviously whatever is about to go down is pretty big...

Walt's neighborhood looks like a ghetto slum in the flash forwards, yet in present times he lives in a really nice middle class neighborhood. What could happen in a year to make a neighborhood drop like that?