Summary - An experimental, analytical, and computational effort was undertaken to examine the effect of confinement on penetration in armor-like steel targets. For the experiments, L/D 10, tungsten-alloy projectiles were fired at 1.5 km/s into 4340 steel cylindrical rounds of various diameters. Penetration efficiencies, as measured by the depth of penetration normalized by the original projectile length (P/L), were determined and the results plotted as a function of normalized target diameter Dt/D, where Dt is the target diameter and D is the projectile diameter. As Dt/D changed from 20 to 5, P/L increased by 28%, although P/L was approximately independent of Dt/D for Dt/D ≳ 15. An analytical model using a modified cavity expansion theory was developed to estimate the resistance to penetration for targets of finite lateral extent. The analytical model shows decreasing target resistance as Dt/D decreases below approximately 30; in particular, target resistance decreases rapidly for Dt/D < 20. Numerical simulations were performed and the computational predictions are in excellent agreement with the experimental results; simulations were used to extend Dt/D between 3 and 78. Plastic strain contours are plotted to assess the extent of plastic flow within the target; the results of the simulations demonstrate that P/L begins to increase when the extent of plastic flow in the target reaches the radial boundary. Copyright © 1996 Elsevier Science Ltd.