How do helmets actually work?

My friend and I were goofing around in a wiped lobby in silo and I asked my friend if I can shoot him in the armor, with a Mac 10, and it did nothing because it was level 4 and then he asked if he can shoot me in the back of the head with a Mac 10, while I was wearing a Mickey mask and it killed me. He was really close to me if that changes anything.