Two of the most popular weapons used in World War II were the Oerlikon 20 mm automatic cannon, and the Bofors 40 mm automatic cannon. When the USS Missouri steamed into Tokyo Bay in 1945, she carried 49 20 mm guns and 80 40 mm. These guns were used throughout the USN, by the Royal Navy, and by the armed forces of several other countries, and on land and in the air, too. American factories produced both by the tens of thousands.
Oddly, neither weapon came from a belligerent. The Oerlikon was Swiss, and the Bofors, Swedish. It is my impression from some reading on this topic that the USA, at least, “pirated” these designs, modified them, and then mass-produced them during the war. After the war, did the USA pay millions in licensing fees or in some way compensate the Swiss and Swedish firms that had patented these designs?