KF Zed gun problem

Hello i have a problem with this weapon. i recover all the pieces of this gun several months ago but i cant buy it. and my others friends yes.



a question.. when all the pieces are collected spawn the achievement? because in my case it wasnt happen that.



A friend gave me the zed gun but when i trie to make the achievement called "Game Over, man" dont work it. dont count the zeds.



what can i do? oh! who can help me?