MPCatchup wrote:With magazine fed weapons belt fed are not far behind so say hello to Ma Deuce or a close facsimile. That takes care of any boarding attempts. Black powder or even brown powder just means more maintenance.
You're thinking blue water.
Any ship eventually anchors. Even a KHVII is at risk from night boarding attacks and lighters full of gunpowder while in harbour. Modern practice seems to prefer at least 30mm for the "sink it, sink it now" anti-boat application.
However, the real application for a rotary cannon is on the brown-water ironclads; those get into outright confined areas and their primary armament has very restricted arcs of fire. A rifle-calibre gatling with a gun shield up on the conning tower would be a big help with the highly vulnerable corners. (Hide in pits, wait, commence boarding off the canal side into the blind corner. High-risk, but easy to imagine the Army of God trying something like that. Enough of those incendiary shells of their could get something bad down the ventilators...)
Larger, 1.5" or so, gatlings up on the casement corners would help even more. Or the Ma Deuce, but both belt feeds and self-loading are tricky; simple in principle, but cranky in practice. It took a lot of operational experience to get reliable machine guns. Gatlings are actually easier.