With the launching of Operation Barbarossa on 22 June 1941, the unanticipated appearance of the KV-1 and T-34 tanks prompted an upgrade of the Panzer IV's gun to a longer, high-velocity 75 mm gun suitable for anti-tank use. This meant that it could now penetrate the T-34 at ranges of up to at any angle. The 75 mm KwK 40 L/43 gun on the Panzer IV could penetrate a T-34 at a variety of impact angles beyond range and up to . Shipment of the first model to mount the new gun, the Ausf. F2, began in spring 1942, and by the summer offensive there were around 135 Panzer IVs with the L/43 tank gun available. At the time, these were the only German tanks that could defeat T-34 or KV-1 with sheer firepower. They played a crucial role in the events that unfolded between June 1942 and March 1943, and the Panzer IV became the mainstay of the German panzer divisions. Although in service by late September 1942, the Tiger I was not yet numerous enough to make an impact and suffered from serious teething problems, while the Panther was not delivered to German units in the Soviet Union until May 1943. The extent of German reliance on the Panzer IV during this period is reflected by their losses; 502 were destroyed on the Eastern Front in 1942.