I've been around plenty Riverine and littoral craft; never saw a hand cranked launcher of any kind. A little surprised with the halting, jerky mechanism, not typical for Honeywell products. Part must be how the chamber and breech work. It's belt fed from left, the empty cases look like still in the belt exiting right side (13 seconds in), so not chambering and extracting in a firearms conventional way. Not a single shot, proper term is repeater, the ammunition quite certain same as M79, except the belt.

The Enterprise CVAN 65 had rifled 20mm deck guns mounted on sponsons to engage surface craft, nothing sophisticated like Oerlikons, Bofors, Crane or Washington Yard etc could make. The shells were impact primers, can't remember the trigger, and don't appear in any scale model of the ship. I think they were made by GM!