Transcolumn dispersion limitations on the separation efficiency of chromatographic columns suggest the need for packing methods that increase bed homogeneity and minimize potential wall effects. Here we address the influence of the slurry concentration in the slurry packing process on the resulting morphology and separation efficiency of ultrahigh-pressure liquid chromatography capillary columns.30–75 μm i.d. capillaries were packed with fully porous 0.9, 1.7, and 1.9 μm bridged-ethyl hybrid particles and 1.9 μm Kinetex core–shell particles. Capillaries prepared with higher slurry concentrations(20–100 mg/mL) showed higher separation efficiencies than those prepared using a low slurry con-centration (2–3 mg/mL). The effect is explained by an analysis of transcolumn bed heterogeneities in three-dimensional reconstructions acquired from the packed capillaries using confocal laser scanning microscopy. The three-dimensional analysis of porosity distributions and local particle size illustrates that beds packed with higher slurry concentrations suppress particle size segregation, however, at the expense of a larger amount of packing voids. In core–shell packings, where only few packing voids were found, the higher slurry concentration allowed for an additional densification of the bed’s wall region, as revealed by a radial analysis of the mean particle distances. Overall, wall effects are attenuated in packed columns prepared with both wide and narrow particle size distributions, which will allow for improved chromatographic performance.