Well, I found this challenge as pure BS, maybe it was an accident or just made on purpose, Treyarch stop bullying your customers
Well, I wondered why the hell I didn't get the medals, but it was because I run classes with ARs and don't have secondary weapon.
So if you want backfire medal, you need a class with a primary and a secondary, then you need to change your primary weapon with the enemys weapon.
Just wanted to point it out, you've got no idea how I felt like crying when I needed my last medal for the title, took an KSG from the ground and killed the KSG user, then I got no medal, then there comes another guy with KSG, kill him and after this I freak out when I hear two KSG shots, lucky me, I jumped and kiled the 3rd guy... There was 3 KSG users and I killed them