Semi-auto only. Limited ammo allowed (100 BBs).

After a majority of the humans were overrun and infected by the zombies, another problem arose:  the zombies quickly ran out of food (humans) and started dying in droves, starving to death.  From this, a new breed of zombies evolved: cannibals.  These super-powered zombies could sustain themselves by attacking and eating other zombies.  A single zombie corpse would sustain ones of these Super Zs for weeks, if not months.  They were also near-impervious to conventional weapons.

Some military official, hiding in a bunker somewhere, made the decision to nuke these colonies of Super Zs.  While this immediately and decisively ended the zombie threat for once and all, the land now lay as a barren wasteland.  Food, water, and medical supplies became the most valuable commodities, with ammunition following right behind. 

The humans who managed to survive both the infection AND nuclear annihilation formed into small, tight groups that fiercely protected whatever commodities they could gather or take from other tribes.