The aim of the paper is to describe a ballistic tolerant approach to evaluate the decrement of the structural integrity of a section of a helicopter tail rotor transmission shaft, made in A16061-T6 aluminum, impacted by a 7.62 ball bullet. The focus lies on the development of a reliable and efficient modelling methodology. Very refined numerical models are used to reduce the experimental efforts and to increase the awareness of the phenomena. Starting from previous work performed by the authors', a numerical model of the impacted shaft is presented and validated by comparison with the results of a large experimental programme. The model is used to assess the variation in the vibration modes and the buckling load caused by the damage. In order to improve the reliability of the model, the mechanical behaviour of the materials of the shaft and of the projectile has been fully considered: flow stress, strain rate, temperature, damage criteria.