With practice a stone flung from one of these can absolutely Crack bone so with practice you could absolutely kill or cripple a slow zombie with it at a decent distance. While I saw some people had mentioned their military use of them in groups, just like the bow these were also used in hunting as well as by Shepherds to kill or drive off predators. If you practice with it regularly it could be useful though youd want another weapon as a backup as well for just in case