Impacts to the head are a common form of body violence and thus a relevant legal medical issue. Biomechanical assessment of injury potential has been traditionally based on qualitative analysis and experience. The aim of this study was to collect benchmark data that would facilitate the assessment of the maximum force in head impacts with longish rigid instruments. Series of measurements were performed with a specially designed modifiable impactor, and the relationship between its inertial properties and its effective mass during the impact was studied. The effective mass was defined as the amount of point mass that would, if exposed to the same velocity change as the striking end of the instrument, produce the same area under the force-time curve as the impactor. The results show that the effective mass decreases from approximately 100% of the total body mass for very short impactors to about 50% for longer (approximately 70 cm) impactors. No influence of the hand/grip force on the effective mass of the impactor was found if it was used in a hammer-like manner;​ other striking techniques can lead to substantial increase of the effective mass attributable to the hand/grip force.