Ixian
Professional Gatekeeper
The cost of upkeep was an issue with the *ship as a whole* in his hands, not specifically the guns.
Pirates rarely fired their guns at all, yes; the upkeep cost was for the ship overall. Privateers likewise rarely fired their guns, acting as state-sanctioned commerce raiders who went after hostile nations' civilian shipping, *not* warships. Basically, privateers use the same limited equipment and tactics as pirates.
In other words: the existence of privateers certainly demonstrates a historical precedent that the Second Amendment *does* reasonably extend to field guns, but they aren't on par with full warships and it was not practical even back then to support a full warship as a private enterprise.
Unfortunately, not actually using the guns is rather antithetical to a militia, as they need to *at minimum* drill enough to be able to smoothly operate a cannon under field conditions.
The East India company maintained a private military fleet, British, not American I know, but it was possible for private companies and people to maintain a force at arms comparable to contemporary nation-states, at least to a degree.