Wait for the two zombies come near you and turn, immediately after they turn, go ahead to the first + sign (wall will close) and stay there just right until the zombies both appear in the tunnel again. Then go straight down without stopping and hide behind the bottom wall, or one of the right ones, depends on how good you are :) From there I am sure you will find it easy.
EDIT: It seem I need to go through the gun and then back through it again in this level to get it.