Only five episodes left...
Okay, so my prediction of what is happening in the flash forward hasn't really changed, but I feel much more sure of it now.
The machine gun is to tie up loose ends, Todd and his new Nazi buddies threatening the lives of Walt's family. He has to be sure they'll all be safe when he's gone. The ricin is to take Marie's advice and kill himself (which I predict will be prevented somehow).
Or maybe the show is so well crafted, it wants me to jump to these conclusions and is going to toss us in an entirely different direction. I've never been able to guess what was going to happen next with this show before. The neo-Nazi casual wiping blood off his shoe as if it were nothing is to let us know he's a real threat, and Marie's telling Walt the only option is to kill himself seems too obvious when coupled with the flash forwards of Walt acquiring a machine gun and then retrieving the dose of deadly poison. These are the obvious reasons for the gun and the ricin, and the show has never been that obvious (has it?).