This paper summarizes the experimental and theoretical status of the optical chronograph developed in the course of ballistic research at the Naval Research Laboratory. The instrument, together with a simple procedure for analysis of data, is capable of yielding the position, velocity, and deceleration of a non‐plastically deforming small arms projectile during the armor penetration process, which lasts from 30 to 150 microseconds. Decelerations have been observed up to 5×107ft./sec.2. The longitudinal vibrations induced in projectiles during impact were observed by a shadow‐photography technique which yields time‐displacement curves from the motion of the trailing face of a projectile during penetration. The precision of the derived decelerations, about 4 percent, is sufficient to permit the evaluation of strain‐rate and inertia effects during high speed indentation by means of conical indenters at strain‐rates approaching 2×105/sec.