I don't think the machine gun is for the Nazis. I don't know what it's for, but I think that's less likely each week.
Who would it be for then? They killed Hank, that was the final straw that ruined his life as he knew it. It was the final nail that killed any chance at keeping his family together. In his mind at least, it was over long before then but his megalomania let him think he could fix it. They took his money, the only thing he cares about - tangible evidence of his empire. He may have severed his fatherly feelings towards Jesse, but there may be some idea of redemption by saving him, or he may be in kill-mode and want to take out the only other person that can make the blue. The top 4-5 reasons to get an M60 all point to the Nazis.