I really don't think that Walt will be using that ricin on himself, Walt isnt the kind of person to kill himself. If he does it will be in a shootout with police where he can at least believe just a little that he has a chance of coming out alive, Walt has always been a fighter. Remember the very first episode? When he thought those cop cars were coming for him, what did he do? He made a quick video for his family and pulled out a pistol, ready to engage the cops in a shootout to the death.
What intrigues me about the ricin and the machine gun is the fact that we know at this point Walter has been found out. Ricin to kill someone in a stealthily fashion so that perhaps no one knows he was poisoned, or used on someone not suspecting it. But considering everyone knows who Walt is who could he be possibly trying to kill?