The paper investigates ball burnishing of openings. A special high-stiffness tool was designed for the purpose of experimentation, allowing the burnishing of 40–120 mm openings. Experiments were performed with aluminium alloy EN AW-6082 (AlMgSi1). The primary goal was to achieve dimensional and geometrical accuracy of the openings. Using a specially designed stiff tool, the openings were widened by 0.06 mm on average, while the roundness and cylindricity errors were drastically reduced, especially at greater ball penetration depths. In addition, the surface roughness was improved by 35%. FEM analysis was conducted to determine the stress field distribution in the workpiece, as well as to approximate the residual stresses after the ball burnishing. Considering the experimental results, further investigation should be directed towards the achievement of high dimensional and geometrical accuracies, as well as increased process productivity.