The shrink guns used by Coreeshi bounty hunters were weapons that could reduce the size of objects that they were aimed at, including living beings. The weapon could be attached to the exoskeletons used by the Coreeshi. The technology was advanced, far beyond the understanding of most species in the Uncharted Territories -- even Sikozu was unable to understand how it functioned.
Primarily the shrink guns were used by the Coreeshi to capture their quarry. However, they are also used as a defense measure -- shrinking any threat to the Coreeshi down to a size where it poses no threat.
Coreeshi shrink guns are seen in an episode of Farscape.