There was a guy on AR15 who went through Argentina's economic crash. His little right up is THE guide to surviving a realistic SHTF scenario. Why? Because he did.
One of his big points was that all the guys talking about AR15's being the go to gun are planning for the wrong thing. As he said, "You're not just going to wake up one day and everything has changed." It will be gradual. Would you be able to go to work now with a rifle on your back? Basically, a handgun is a must in the city because it is concealable. A rifle is for protecting your home.
That being said, if we are talking the mother of all SHTF, then the AR15 is a good choice because it is compatible with what the military is using. Easier to scavenge bodies for ammo and parts if you are using the same rifle. Plus the odd raid on supply convoys can be helpful.