Well, yes, that's the point I'm making. The US swung wildly from "privateering is an inherent right we will never give up" to "privateering is illegal and we propose to hang captured privateers as pirates" to "privateering is an inherent right we will never give up" depending on whether they were looking at actually facing privateers. The states which went ahead and committed to not having privateers straight off based on a legal quid pro quo (you don't have to face privateers if you are willing to give them up yourself, see "land mine treaties" for an analogy) are on rather firmer ground for having consistent principles.Correct me if I'm wrong, but doesn't America still today reserve the right to issue letters of marque? I recall a couple articles put out by the US Naval Institute on the subject a year or so ago.
Are there any actual cases of such an operation taking place in that period, especially on the open sea? The only one I know about was a customs scam in the War of 1812, though I've not exactly looked and only know that one because it's funny.Also keep in mind that privateers of the 1860s didn't necessarily have to carry a lot of armaments ... merchant ships by that time were already starting to drop the practice of carying artillery as had been common in the preceding century. A supply of small arms was sometimes all you needed. Look at the way Somali pirates have attacked unarmed merchants in the late 2000s ... not an artillery piece in sight, just some AKs. Replace those AKs with Enfield muskets and revolvers, and you've got a privateer that can take on a merchantman that has no cannons aboard ca. 1860s.
As for guns left over from 1812, yes, I should clarify the state has to provide the guns in general. I'm not aware of much in the way of cases of a privateer purchasing overseas cannon, either (in the Napoleonic wars when there was a lot of privateering) though ditto for my not having looked.