The name Pak 40 refers to a small series of very popular German WWII-era anti-tank guns. The most common was the 7.5 centimeter (or 75 millimeter). It was developed by Rheinmetall in 1942 and was used until the end of the war in 1945.
In the film
During the Beetfield Assault, there were two Pak 40s hidden in the treeline. Both fired multiple rounds on the incoming 3rd Platoon only for their shots to miss, resulting in Fury taking them both out.