When exploring with a friend, sometimes, I can't or they can't hear when a gun is shot. I had my friend 5 feet from me when i shot at a zombie with a rifle, the bk-18, and they heard nothing. A horde of zombies rushed us and I switched to the bk-33 shotgun and kept firing. The entire time my friend was meleeing, wondering why they were rushing us when we were being stealthy. It's happened multiple times at random.