In addition to the high explosive round, the 75 mm gun also had an effective canister round that functioned as a large shotgun and a white phosphorus (WP) shell originally intended for use as an artillery marker to help with targeting. M4 tank crews discovered that the WP shell could also be used against the Tiger and Panther. When the burning white phosphorus adhered to the German tanks, their excellent optics would be blinded and the acrid smoke would get sucked inside the vehicle, making it difficult or impossible for the crew to breathe. The fear of the fire spreading inside the tank, would sometimes cause the crew to abandon the tank. There were several recorded instances where white phosphorus shells defeated German tanks in this fashion.