[@Wombatallion]

As an fun little anecdote, in an high security prison in Germany, two Inmates build themselves pistols and shotguns out of all kinds of stuff they could get their hands on.

Making bullets is also quite easy with the right know-how, as the "base" material would be abundant, even in a stripped and looted city! Then again, the Know-how would be the true limiting factor, and the gunsmiths most likely be as sought after as skilled doctors! 

Just my two cents input ^^ 

As for my ideas for factions:

[u]Metro-Runner:[/u]
The Underground makes for a perfect trading platform right below the frontlines of gang-warfare. If a single fraction would rule the tunnels below London, they would rule the trade, traffic and majority of the wealth. (A rather peaceful gang, looking to expand their control of the tunnels, and trying to establish themselves as a "neutral" ground. They wouldnt start off with total control of the metro, but rather a part of it, trying to expand.

[b]Crimson Dawn:[/b]
Founded by an charismatic leader, a mysterious cult has taken control of Saint paul's cathedral, who declare that "Thou what thou wilt" is the only law of the new world, except for "Thou what thou are told by thy Prophet"! Growing rapidly, as some claim that the prophet is Aleister Crowley reborn, they violent debauchery and ceremonies are outlandish for some, and horrifiying for most! 
(Occultist Nutcases, who would make a fine enemy for any DEUS VULT faction ^^)