The S-51 was an attempt to make the heavy 203 mm howitzer M1931 (B-4) mobile by using the KV-1S chassis. While testing on April 1944 there where serious problems with the high recoil of the gun which couldn't be fixed. In FHSW the S-51 can be used for close support of tanks and infantry or simply as artillery to fire from great distances using the binoculars.