You often find them attached to rocks or slabs, right at or just below the surface of the sand. That's the transition area between the sand dwellers and the rock dwellers, so they may eat both sand-dwelling bivalves as well as other snails (or even barnacles). However, Radwin & D'Attilio said this:
"Paine (1963) stated that Chicoreus florifer has the most specialized diet of the eight predatory species he studied. Radwin & Wells (1968) reported that...(several species)...exhibited strict prey specificity, whereas Muricanthus fulvescens, Chicoreus florifer, Urosalpinx perrugata, and U. tampaensis were found to be less discriminating in choice of prey."
You might track down those papers at the local library, or one of the university libraries and find out exactly what they said, but the latter paper sounds like they might eat whatever molluscs they can catch.
Cheers,
Don