Mission 7 - All Roads Lead to Hell - Chapter 4
From your spawning location, run straight passing a Heavy Machinegun and you will see lines of about a hundred zombies standing near explosive barrels. Simply shoot a barrel with the Heavy Machinegun, restart checkpoint, and repeat. Your challenge for the chapter is dismember 200 arms with HMG, so you'll have an idea how many your getting each run. Hit the start button and click on the bottom right to Track the Challenge, it's called "Bloodbath".