<p>We present a detailed numerical study for the dynamic compression of aluminum-alloy specimens in the Kolsky-bar system. The purpose of this work is to follow the full compression process, through 2D and 3D simulations, from the impact of the striker bar to the deformation of specimens with various aspect ratios. The stress–strain curves of these specimens were calculated through the conventional Kolsky-bar relations, using the strain–time histories at two locations on the bars which match their locations in our Kolsky-bar system. This work highlights the effects of the specimens’ inertia and their friction with the bars. Special attention is given to the non-uniformity of the stresses in the specimen, due to friction at the specimen/bar interfaces. These non-uniformities are shown to depend on the aspect ratio of the specimens, and they also lead to non-uniform distributions of the temperature in the specimens. The simulation results are used to determine an optimal aspect ratio of about 1.0 for metallic specimens under compressive loading in Kolsky-bar systems. We also show that by simulating these Kolsky-bar tests, one can follow the different failure modes of ductile and quasi-brittle materials.</p>