So people who live in urban enviornments are hopelessly screwed?
I live in downtown San Francisco in a neighborhood favoured by mendicants and other n'er do wells; so if it ever came down to a lawless-type situation, I am asking my local gangbangers for a deal on whatever they have lying around. It is they who'll be doing my stockpiling of ammunition.
In all seriousness though, it comes down to an issue of space. In the spirit of rugged American individualism, it's assumed that everybody owns a house with ample storage space; combined with the inevitable fact that a car is required material, then you have the logistics and transport covered.
San Francisco on the other hand, like NYC, is an "old-city" with a high-density downtown that precludes car-buying. Plus, since I probably pay more for rent than most folk here do for their mortgages (and get less in return), it simply is not possible to store ninety-days worth of anything in my apartment; so I'm pretty much reliant on city services in the event of a major anything. As has been discussed here and elsewhere, it's believed that those'll be overwhelmed in the initial response (as was witnessed during 11 September and the Aum Shinrikyo sarin gas attacks).
But this is all besides the point. The point is, modifying what is essentially a suburban-rural survival plan to accomodate urban dwellers. To wit, this is what I've come up with (mind you, it's late and I'm tired):
1.) Splitting your cache. I for one keep a moderately sized disaster preparedness kit in my apartment; it won't last ninety days, but if used properly it should last two weeks. In all sense, that should be enough to last until I get to the main stockpile. Since space is such a premium here in the Bay Area, it would be optimally located further down the Penninsula in, say, a self-storage unit(?).
I know the idea is to have it all with you, but given space requirements and the possibility of a last-minute EVAC, this seems definitely feasible, if not viable.
2.) Since everything tends to happen in close-quarters in an urban enviornment, I would second TR's recommendation of the shotgun since the idea is home-, personal defense. I doubt you'll be shooting at anything beyond 25m in the city, unless you were some sort of Evel Antagonist. I would also round it out with a revolver and a bat, an aluminum one preferably.
3.) A well planned EVAC route. Since congestion is a city-dweller's worst nightmare, it would be vital to have multiple routes out of the city planned out, falling back to each one if the previous one went FUBAR.
Anyway, that's just a couple thoughts I had. Feel free to expand upon them, or shoot them down. Or, ignore them completely. I don't mind.
-KS