Even with the first tanks, artillery officers often used field guns depressed to fire directly at armored targets. However, this practice expended much valuable ammunition and was of increasingly limited effectiveness as tank armor became thicker. This led to the concept of anti-tank guns, a form of artillery specifically designed to destroy armored fighting vehicles, normally from static defensive positions (that is, immobile during a battle). The first dedicated anti-tank artillery began appearing in the 1920s, and by World War II was a common appearance in most armies. In order to penetrate armor they fired specialized ammunition from proportionally longer barrels to achieve a higher muzzle velocity than field guns. Most anti-tank guns were developed in the 1930s as improvements in tanks were noted, and nearly every major arms manufacturer produced one type or another. Anti-tank guns deployed during World War II were manned by specialist infantry rather than artillery crews, and issued to infantry units accordingly. The anti-tank guns of the 1930s were of small caliber; nearly all major armies possessing them used 37mm ammunition, except for the British Army, which had developed the 40mm Ordnance QF 2-pounder. As World War II progressed, the appearance of heavier tanks rendered these weapons obsolete and anti-tank guns likewise began firing larger calibre and more effective armor-piercing shells. Although a number of large caliber guns were developed during the war that were capable of knocking out the most heavily armored tanks, they proved slow to set up and difficult to conceal. The latter generation of low-recoil anti-tank weapons, which allowed projectiles the size of an artillery shell to be fired from a man's shoulder, was considered a far more viable option for arming infantrymen. The RPG has its roots in the 20th century with the early development of the explosive shaped charge, in which the explosive is made with a conical hollow, which concentrates its power on the impact point. Before the adoption of the shaped charge, anti-tank guns and tank guns relied primarily on kinetic energy of metal shells to defeat armor. Soldier-carried anti-tank rifles such as the Boys anti-tank rifle could be used against lightly-armored tankettes and light armored vehicles. However, as tank armor increased in thickness and effectiveness, the anti-tank guns needed to defeat them became increasingly heavy, cumbersome and expensive. During WW II, as tank armor got thicker, larger calibre anti-tank guns were developed to defeat this thicker armor. While larger anti-tank guns were more effective, the weight of these anti-tank guns meant that they increasingly were mounted on wheeled, towed platforms.