Here is my heirarchy of SHTF weaponry:
1) AK-47: Cheap, reliable, cheap ammo, and if you live in a city like me, you won't be taking 200+ yard shots anyway. Additionally, slidefire solutions just came out with a "bump-fire" stock that can be fitted to any stamped AK-47 receiver, making it equivalent to fully automatic. Totally BATFE legal....for now.
2) Saiga 12-Gauge Semi-Auto Shotgun: Great for city combat; capable of holding a 25 round barrel drum, turning it into the best street-sweeper in town. Very reliable, and you can get great deals on cheap Sellier-Bellot 9 pellet buckshot on cheaperthandirt.com for like 25 shells for $10. In a riot situation, this street sweeper will seriously thin the crowd.
3) PTR-91 KF: Surprised? Greatly improved reliability over the ammo-finicky models of the early days. This HK-91 clone in .308 caliber will give you the ability to "reach out and touch someone" at 400 yards with iron sights, and 650 with a scope, on the chance you can make it out of the city to the countryside (where a bit more range will be desirable). Priced at about $900-1100, the new ones run well on cheap .308 surplus ammo, which is a huge benefit, given the average cost/round of domestic ammo. Did I mention it might be the only gun meaner looking than an AK-47?
4) Russian SKS. Surprised again? You can get these on gunbroker.com for $400, and they use the same cheap 7.62x39 ammo as the AK-47. Limited in range to 200 yards, but if you need to arm a handful of recruits to help you defend the fort, you can get 5 of them for the cost of one average priced .308. Good to have a few of these in the safe to pass out.
5) Gammo pellet rifle: Stealth hunting will be important.
6) Ruger 10/22: Cheap ammo; you can get 550 rounds for less than $20; the ammo itself could be a currency, but the gun is easily the most accurate "out of the box" .22 caliber rifle in the world. And at $200+ why wouldn't you have one? The availability of sub-sonic ammo also makes for the possibility of a tactical close range sniper if the gangs hit the city (i.e., You won't want them to know it is you shooting, but would be nice to thin the herd a bit).
7) Handguns: Anything bigger than .32
Just one man's opinion!