How come Mumei's pistols can kill Kabane?
As you know and heard of, Mumei is a kabeneri (half- human and half- kabane). And apparently, once you become a kabeneri, your abilities as a human increases vastly. For example, your speed and strength increases. Remember, when she talks with Ikuma inside the train, she says "if I remove the ribbon around my neck, I could use my real abilities". But she doesn't usually removes it, because as a side effect, she get's exhausted and sleepy. And also Mumei is seen in the anime as one skilled with martial arts. Therefore, due to her enhanced abilities- thx to kabane virus, it helps her to get Insta kill with her pistols. And also the ammunition which she uses are normal bullets and they are limited. 

I think this helped you in finding your answer and for more information about her, visit the link below.

http://koutetsujou-no-kabaneri.wikia.com/wiki/Mumei