One in the chamber:

Players will have one bullet in there gun, shooting a zombie will replenish the bullet (it would be a one shot kill situation like the knife challenge). If they missed their shot, the player would down. (if too harsh of a punishment, then they would be one hit away from down for the rest of the round, but it replenishes the bullet)

Round: N/A

Sticks and Stones:

Zombies can only be killed with ballistic knives and semtex, both of which are given to the player at round start. At round end their original weapons are given back.

Round: < 25

Not really a challenge suggestion, but something I think would be cool in the future is if each map had their own individual challenges like the bo4 version. Of course that's assuming it's possible.