Guns are mostly useless against zombies. Humans, however, die to a single headshot 99% of the time. So that's helpful, especially when the alternative is charging a shooting opponent.
Suiciders are easy to kill with pistols, and killing them at range is a good idea, since they explode killing anybody in a couple meter radius.
Shotgun headshots are instant kills against anything, making them useful against special zombies. You may find ammo blueprints, making firearms more viable. Reportedly there are enough raw materials to be found to allow you to rely on crafted ammo for most of the game. There are also repeatable quests that reward some ammo: rifle ammo.
The fact that top gun mods are about 5 times worse than top melee mods puts another nail in her coffin.
Her fury skill, which gives you a revolver with unlimited ammo for a few seconds, can be useful, though other characters have capable fury skills as well, especially Xian.
Bottom line: guns are situationally useful, melee is still the workhorse of the game. Purna's skills may remedy this somewhat, but other characters are perfectly capable of using firearms effectively, making her a poor choice.
PROTIP: dropping a gun will empty its clip into your ammo pool, so you can benefit from picking up and dropping subpar firearms. If your friends have some guns they aren't going to use, have them trade the guns to you and drop them, getting some extra ammo.