Thank you for the followup. The siege was one of the alternatives I considered earlier, but it didn't fit your description. You wrote "cut off from supplies, power was lost, but yet there was no invasion or shelling".
The total blockade of Sarajevo started on 2 May 1992, and included sending in armored columns to try to take the city (https://en.wikipedia.org/wiki/Siege_of_Sarajevo ) and shelling (see http://www.bosnia.org.uk/news/news_body.cfm?newsid=2687 ).
There appear to have been several types of gangs. The Selco link you gave talks about gangs in the size of 15-50. These appear to be different than the organized "drug gang" style of gangs you mentioned before. Selco writes that being alone, even armed, wasn't enough. He was part of a large family of 15, and that being in a group was the key to survival, not so much arms alone.
Do note that he also wrote that there was a lot of grey, not white on black. I don't think there's a good lesson from this. Having more arms might change the balance of which group gets which resources, but there's no good or bad side in those winners, only "me" instead of "you."
A better example of what you want is perhaps http://www.nytimes.com/1993/10/22/world/gangs-in-sarajevo-sp... , which says that "armed gangs have profited from the disorder of war to turn whole neighborhoods into personal fiefs."
"The gangs control the thriving black market, which accounts for virtually all the trade in food, alcohol and vehicle fuel in the besieged city. Working with similar gangs operating on the Serbian side of the siege lines, the gangs run a nighttime smuggling operation that brings truckloads of contraband over the bridges across the Miljacka that separate the Serbian-held suburb of Grbavica from the center of Sarajevo. ..."
"The gangs' power is so great that the leaders of the Bosnian Government and army said recently that they dared not challenge them directly for fear of setting off an internal war in Sarajevo that would weaken the city's defenses. Bosnian Army commanders have acknowledged that key sections of the front lines around the city are under the control of militias loyal to the gang leaders and that challenging the gangs could cause the militias to abandon their positions."
From https://en.wikipedia.org/wiki/Ismet_Bajramovi%C4%87 , "When the war began, criminal groups were among the first to offer resistance the Yugoslav National Army besieging Sarajevo." Other criminal/military defenders include Ismet Bajramović include Ramiz Delalić, Jusuf Prazina, and Mušan Topalović.
That said, I'm still not clear how more pistols and rifles would have changed what happened in the city, in regards to how people lived. (Weapons were useful in defending the city, and the tunnel was able to supply more of those. That, however, is a quite different topic than what you suggested.)
I can see how this is good example of "shit hits the fan". I don't see how it's at all a good example of your original scenario, which was:
> a disaster happens, so police response times have lapsed, dropped to days. Not everyone is stranded on their roof, tons of people have food but the real problem is that now heavily armed and well organized drug gangs are able to operate with impunity, and brutality, seizing whatever they wish (including the food)... It's a power-grab situation for a few days, or for however long the power stays off, or it would be, if combat veterans and ordinary Americans weren't so well armed.