Naticid gastropods provide a highly tractable system for exploring ontogenetic changes in diet because the telltale boreholes they leave behind in their hard-shell molluscan prey can be linked reliably to naticid size. In this study, ontogenetic changes in diet and size selectivity across 7 common prey species of Lunatia heros (Say, 1822) were explored using a forensic approach based on beach-collected shells from the eastern coast of Nova Scotia, Canada. Laboratory experiments were also undertaken to assess the relationship between predator size and outer borehole diameter (OBD) for 3 of these prey species. The results confirmed a strong relationship between OBD and shell length for L. heros and demonstrated that this relationship was consistent across prey species varying in shell thickness and composition. Analyses of drilled, beach-collected shells revealed that L. heros clearly differentiates among prey with respect to size and species, with ontogenetic changes in prey selection appearing to reflect shifting predator—prey size relationships. Among prey species, larger predators demonstrated the capacity to select larger prey species and avoid smaller prey taxa. Similarly, within prey species, larger predators preyed on larger individuals of a given species, but also exploited a wider selection of prey sizes than smaller moonsnails. The moonsnail L. heros also exhibited borehole site specificity for both bivalve and gastropod prey, but did not discriminate between left versus right valves when drilling bivalve taxa. The results of this study are important for understanding the changing role of L. heros across its life span within soft-sediment communities. This research should now be broadened to encompass the full suite of prey species targeted by this drilling predator.