An APS Underwater Assault Rifle was used once as one of those exotic 15 minutes of fame kind of guns that makes a "guest" appearance in an episode.
This rifle was given to Revy from Dutch at the end of one episode for an underwater mission to retrieve a precious painting from a sunken Nazi submarine that ended up in the South Pacific. Revy fired the weapon inside the sub against the Neo-Nazis that also were after this prize. Since the weapon was designed to function both above and under water, the weapon was still able to function properly in the air-filled submarine. She later fired the gun underwater at two divers on the surface next to the pod, which is what the APS Underwater was designed for.