A model for rigid projectile penetration and perforation of hard steel and metallic target plates is suggested. The intended application is in computer programs for assessment of effects and vulnerability for complex targets, such as tanks, fighter aircraft and naval ships. The target material resistance to penetration is assumed to be constant, corresponding to lateral displacement of the material along the projectile trajectory, until the front end of the projectile is sufficiently close to the rear surface, whereupon the remaining volume of target material in front of the projectile is crushed and forms secondary fragments. At the penetration depth where this occurs the force required for crushing equals the force required for continued lateral displacement of the target material. The secondary fragments are ejected with the same velocity as the projectile.