Eroding projectile penetration is described with a model that satisfies fundamental requirements, namely mass continuity, energy conservation and momentum conservation. Target material is deformed according to established quasi-static elastic-plastic theory. Projectile deformation corresponds to melting from the front end and deposition along the hole wall while erosion is not disturbed, namely as long as the velocity and hence the hole diameter is sufficiently great so that there is room fro the eroded material between the hole wall and the remaining projectile. Towards the end of the process erosion becomes disturbed whereby friction between previously eroded material and the remaining projectile increases retardation of the latter.