Reamers are intended to smooth or enlarge the holes in beads. Generally the reamer bits are made with diamond-dust coated surfaces in different shapes to suit the task, though Pearl reamers are cork-screw shaped. Available in manual twist reamers or battery operated if you need to do a lot of reaming.
When using a reamer be sure to hold the bead under water in order to provide lubrication - reamers are delicate items and need to be treated with care. Never push the reamer into the hole - let them turn and find their own way, constantly removing the tool to ensure there is no build-up of debris.