How would small sailing ships armed with twelve or eighteen guns be rated?
Short answerBoth would be unrated brigs of some type; the exact label would depend on rig, hull form, and purpose.
NotesBoth proposed vessels fall below the formal rated-ship system despite carrying twelve or eighteen guns. They are broadly brigs, although that is only the beginning of the classification. Rig, hull form, and purpose could support a more specific description, including a yacht in the right configuration. Gun count alone cannot determine the type of a small sailing warship.