[JamesP81]

However, perhaps some of the private sailing ships had cannon?

They were called privateers.

[geopyg]

Did they still have legal privateers in the late 1700’s?

Off topic - Captain Kidd was actually a privateer (not a pirate) back in the 1600’s and hired by NY businessmen and backing of the king to go after pirates. I read a great book on the “TRUE” story of Captain Kidd, and it was very well documented and he followed the rules of engagement of being a privateer, but took the fall when he was too agressive. One was taking a ship flying a French flag which was okay, but the ship was actually Dutch or something and carrying Indian textiles. He was legal in taking it, but pissed the Dutch and Indians off - who were friends with the British. There are actually the legal documents (shipping records, seals, etc.) still in London that prove his innocence but were not brought up in his kangaroo court trial.

[USMCVet]

"However, perhaps some of the private sailing ships had cannon? They were called privateers."

Or just armed merchantmen that didn't particularily want to be taken as a prize by a privateer..