The experiment reported in this paper was designed to obtain data on the dynamics of a nonrotating, conical‐nosed projectile penetrating randomly‐packed sand. Positionversustime measurements for the projectile in sand were obtained by means of a photographic‐electronic chronograph developed for the purpose. The striking velocityv0of all rounds was about 700 m/sec. The negative acceleration of a 5‐in. long, 0.50‐caliber, 80‐gram projectile was found to be roughly expressible by the equation−dv/dt=&agr;v2+&bgr;v+&ggr;where the coefficients &agr;, &bgr;, and &ggr; are positive constants. This general relation includes as special cases the conventional penetration formulas of Robins‐Euler, Poncelet, and Re´sal. A new theory of penetration is proposed based on the equations:−dv/dt=&agr;v2,v0>v>vc;−dv/dt=&bgr;v2+&ggr;,vc>v>0where the coefficients &agr;, &bgr;, &ggr; are positive constants and &agr;<&bgr;. An abrupt transition in the drag force that occurs at the critical velocityvcof about 100 m/sec is believed due to transition from inelastic to quasi‐elastic impact.