In 1943, the thick armor of the new German Tiger and Panther tanks had rendered the F-34 gun obsolete, and experiments were conducted to find a replacement. The better-penetrating 57 mm ZiS-2 high velocity antitank gun was installed on some T-34/57 tanks, but the smaller-bored gun couldn't fire an adequate high explosive round for general use. A more heavily armoured T-34 prototype was built, the T-43, but it was still vulnerable to the Tiger's 88, and its mobility suffered too much from the weight of armour. In the end, the F-34 was replaced by the D-5T and ZiS-S-53 85 mm calibre guns on the T-34-85 tank, and by 122mm and 152 mm guns on the new IS-2 heavy tanks and in the Soviets' casemate-type, turretless tank destroyers. Some F-34 guns were also installed in Lend-Lease M4A2 Sherman tanks, known as the M4M. The L-11 gun was mounted on the initial T-34 Model 1940 medium tank and the KV-1 Model 1939 heavy tank. The F-32 was on the KV-1 Model 1940. Subsequent models of both tanks mounted the F-34 until it was replaced by 85mm guns in the T-34-85 medium tank and KV-85 heavy tank.