A numerical and experimental assessment of the compression test in the split Hopkinson pressure bar (SHPB) has been made. The DYNA2D finite element code was employed in the numerical part. The aim of the work was to establish the influence of an important reduction in the specimen diameter on the results. To this end, several numerical experiments were carried out with different diameters. Experimental measurements using the SHPB technique were also performed. The material studied was the 7017 T73 aluminium alloy. In the simulations, stress histories were registered at different places’ in the incident and output bars, as well as in the test specimen. Numerical simulations show important three-dimensional effects in the SHPB, increasing for smaller diameters. Experiments show the sa me tendencies evinced by the numerical simulation. Care must be taken to minimize them to achieve the desirable uniaxial stress condition on the specimen.