capable of blowing the turret completely off a Tiger tank. A direct hit usually destroyed or damaged the target's tracks and suspension, immobilizing it. While the low-velocity 152mm shell did not generally penetrate heavy armor, it frequently killed or severely wounded the crew through spalling (splintering) inside the hull as well as injuries caused by blast concussion. Due to the brittle nature of German tank steel during the latter stages of the war, it was common for impacted armor plates to shatter from the concussive blast, or for weld lines to break, allowing the entire armor piece to fall away. Surviving crew were often left with an immobilized vehicle which had to be hurriedly abandoned before being destroyed. For anti-tank operations following the July 1943 Battle of Kursk, armour-piercing ammunition was developed, with an eye towards giving the howitzer a more traditional anti-tank capability. However, these rounds were expensive, in short supply, and only moderately more effective than the standard non-penetrating high-explosive round. As a howitzer the ML-20S exchanged velocity and accuracy for throw weight and distance, and was not intended to compete with true anti-tank guns. Sometimes the concrete-piercing ammunition was used for the anti-tank role. A primitive shaped charge ammunition, with a shell, was also developed. It had a maximum penetration of 250 mm of RHA at 90°, but it was not used during the war. The ISU-152's 90 mm of sloped frontal armor, in contrast to the SU-152's 65 mm, provided excellent frontal protection from the 75mm KwK 40 gun of the ubiquitous Panzer IV and StuG family at all but the closest ranges, while also forcing the original Tiger I, with its vaunted 88 mm KwK 36 gun, to close to medium ranges in order to successfully penetrate the vehicle, negating its traditional long-range superiority and putting it within effective range of the Soviet T-34-85 medium tanks The ISU-152 was not a true purpose-built tank destroyer. It had a very low rate of fire compared with specialised tank destroyers such as the German Jagdpanther or the Soviet SU-100, which could manage a brief burst of five to eight rounds per minute. However, prior to the introduction of the SU-100 it was the only Soviet armored vehicle capable of tackling the German heavy tanks with any kind of reliability, and its ability to satisfy multiple roles meant it was produced in far greater numbers than the SU-100.