How about at some point they invent the spear, like oh, every other civilization? Zombies have two weapons, their arms, and their teeth. If you invent a weapon with a longer reach than that, you are invincible.
I wonder if a crossbow, a samurai sword, or sawed off shotguns would work in this world?
Agreed. They are relying too much on knives at short range and guns with ammo running out. Spears let you destroy the brain, while at a safe(r) distance.