Killmarks 101

Could someone from the community give me a brief explanation on killmarks. I understand that the ability to mod killmarks isn't possible, but my friend showed me that his killmark was different from mine and I'm wondering is it possible to change the default killmark. Is it because of his turtleshell gun or is there another way? Heres the link: http://postimg.org/image/y96p22u2v/