SHTF Gun? What kind of S and what sort of F??
There are far too many factors to just say "This one" for all cases.
I am first a foremost a gun guy, but they are just tools....some are better for certain things then others. I always liked the quote:
"When you did to do something you go to the toolbox and get the right tool for the job."
I've gone around and round on this one before with some of my buddies. Frankly, the average person would be very well armed with a 30-30 carbine and leave it at that. All the whiz-bang tactical crap is a lot of fun, and the gaming is a lot of fun, but for the most part I doubt this foolishness will stack up in the real world. People tend to forget when they are getting super-ninja para-military that the military does NOT operate in a vaccum. A solider is a member of a unit, team, squad, platoon, and so forth. There is other armed and trained people there with him...there are complimentary arms (grenade launchers, SAWs), there are support arms (mortars, SMAWs) and heavy support in the form of air, armor, and artillery. Not to mention there is a huge logistic chain to keep the troops fed, housed, armed, and otherwise supported. Inteligence gathering and a Command structure to make sense of it (I know, stop laughing) and implement plans.
A squad is not alone in any sense of the word. If I recall correctly the logistics chain equated to something along the lines of: Eight men in support for every one that goes into combat. Unless you have that kind of all around support then....YOU SHOULD NOT BEING TRYING TO USE MILITARY PHILOSOPHY!!
Anyone that thinks they are going to get a couple of their buddies together after SHTF, men they have never trained with (and prehaps have never really "trained" at all) and go forth to perform house clearing and urban combat need to have their heads checked.
Having said all of that......I'll take my FAL.