World War II saw a leapfrog growth in all areas of military technology. Battlefield experience led to increasingly powerful weapons being adopted. Guns with calibres from 20 mm to 40 mm soon gave way to 50 mm, 75 mm, 85 mm, 88 mm, 90 mm and 122 mm calibre. In 1939, the standard German "panzer" had either a 20 mm or 37 mm medium-velocity weapon, but by 1945 long-barrelled 75 mm and 88 mm high-velocity guns were common. The Soviets introduced their 122 mm in a turreted heavy tank series, the Iosef Stalin tanks. Shells were improved to provide better penetration with harder materials and scientific shaping. All of these meant improvements in accuracy and range, although the average tank had to grow as well to carry the ammunition, mounting, and protection for these powerful guns.