hi TheDakoya,
this is what it says for the challenge:
Found Kills
Pick up a weapon and get 5 kills without dying, with all kills coming from that weapon.
I am assuming that means you pick up a weapon from an enemy and get 5 kills (which is a bloodthirsty) without dying and all the kills have to come from that weapon. Meaning you can't used your secondary weapon, lethals or scrorestreaks because those kills won't count.
.