The computer-model MUMS is a model for simulation of duels between threatening torpedoes and submarines or surface ships equipped with underwater countermeasures for protection. With earlier versions of the model only soft kill countermeasures could be simulated. This report concerns the addition of an antitorpedo-torpedo. The modelling of the target-seeking, target-following, guidance and motion of the antitorpedo-torpedo is described. Changes and complements of the program-code are also described.