Note: this only works on grief.

Buy the M14 off the wall, then open the door of the building where the second mystery box usually is.

Go upstairs and wait until a zombie is climbing through the upstairs window.

When it is climbing through, shoot the M14 once, then knife, then shoot again.

If done absolutely correctly then the zombie should walk out off the ledge and across the map and back again before coming up to attack you.

[unconfirmed]