that debuted in 1940. French doctrine was for their infantry to let enemy tanks pass through then stop the accompanying enemy infantry leaving the unsupported tanks to be engaged by anti-tank guns deployed in three echelons. The issue of 58 guns per division provided 10 guns per kilometre of front which was expected to be able to deal with enemy tanks at a concentration of 50 tanks per kilometer. In practice the German invasion of France concentrated tanks in select divisions at up to 100 per kilometer. Introducing improved ammunition and increasing muzzle velocity initially helped compensate for their mediocre performance, but small-caliber anti-tank guns clearly would soon be overtaken by yet more heavily armored tanks. Medium-caliber guns in the 40- to 50-mm range began to appear, some of which simply used rebored 37-mm barrels. Although they, too, were soon approaching obsolescence, most remained in use with infantry units until the end of the war. Anti-tank guns remained ineffective against sloped armor, as demonstrated by an incident in 1941 when a single Soviet T-34 tank was hit more than 30 times by a battalion-sized contingent of German 37 and 50-mm anti-tank guns. The tank survived intact and was driven back to its own lines a few hours later. This helped earn the Pak 36 the moniker of Panzeranklopfgerät ("tank door knocker") because its crew simply revealed their presence and wasted their shells without damaging the T-34's armor. Anti-tank gunners began aiming at tank tracks, or vulnerable margins on the turret ring and gun mantlet, rather than testing their lighter cannon against bow and turret armor. These difficulties resulted in new types of ammunition being issued, namely high-explosive anti-tank (HEAT) and armor-piercing discarding sabot (APDS) projectiles. Towards the end of World War II, armor plating became still thicker, with tanks such as the Tiger II being fitted with armor over 100 mm in thickness, as compared to the 15 mm more typical in 1939. This prompted the development of a third generation of anti-tank guns, large-caliber pieces in the 57- to 100-mm range. The British Army adopted the Ordnance QF 6-pounder and Ordnance QF 17-pounder, which were then considered great advances in firepower, and the Wehrmacht fielded the even larger 7.5 cm Pak 41 and 8.8 cm Pak 43. While the early 37-mm anti-tank guns were easily concealed and moved, the large-caliber weapons available late in the war required equally large