Mercop,
I think that there would be a huge difference between just trying to survive in a war torn environment or in the wake of a disaster and actually fighting as a guerrilla or insurgent.
For survival purposes, I'm of the opinion that a couple of good handguns, (one full size, one mousegun), and a good fighting long gun, be it a shotgun or a rifle is probably all you need. In that situation, you're trying to avoid combat if at all possible. Being armed would be essential, but making a big show of it would likely bring the unwanted attention of whatever authorities, (police, military, gangs, mafia, etc...), were still existent.
As far as being a guerrilla, I think the Iraqi's have demonstrated how a lot of it is done on a modern battlefield and they seem to rely on IED's, close range sniping and assassination. It seems that when these groups go head to head with professional soldiers though, they get their butts kicked.
I'm sure a Mosin Nagant would be adequate for kills out to 300 yards or so, but it would be one of my last choices in a firearm if I actually had to go into harms way in the 21st century. They're reliable and powerful, but they don't mount optics well and they're certainly not well suited to CQB.
I've got five of them in my safe however and I do like taking them out to the range.