By the beginning of World War 2, the thickness of armor on tanks had increased significantly. So much so that the majority of previous Anti Tank Weapons were now ineffective.
The Germans were quick to develop the AP40 series of rounds - a tungsten carbide core, wrapped in light alloy.
The lighter round achieved higher muzzle velocities whilst the tungsten carbide core improved armour penetration up to 30%. A 3.7cm AP40 round was capable of penetrating 2.8inches (72mm) of steel at 220yards (200m).