Analytical expressions for the statistical distributions of mass and number of natural fragments from warhead casings are available in the literature. These formulae can be combined with models for estimating the terminal ballistic effects of preformed fragments. The models are based on an analytical and empirical description of the fragment velocities and projection angles of warheads with performed fragments. These modes are incorporated in the expert system WDES (warhead design expert system) that is used for analysing and designing fragmentation warheads. The statistical distributions are verified by analysing the experimental results from a warhead (342 mm caliber, 527 mm length) and comprising experimental and theoretical distributions. The velocity and angle distribution and the terminal ballistic efficiency of the natural fragments are simulated with the expert system WDES. A procedure is given where the natural fragments are substituted by preformed fragments whose ejection properties and penetration capabilities can be calculated.