Its actually a 380mm naval design rocket mortar and not a "500mm gun". It held 14 rounds of this ammo including one in the barrel. It was realoded from inside and intended for street fighting.
This was not intended for antitank action.
As for the Sherman story maybe you heard of the one about the Jagdtiger and a Sherman.
Basically this Jagdtiger spotted a Sherman some distance out but before he could fire on him the Sherman dissapeared behind a house. No problem! The Jagdtiger took aim, fired and saw the Sherman turret fly over the roof. A hit! What makes this so amazing is the "some distance out", the Jagdiger gunner estimated the range to be over 5000 meters!!!