Sir,
In the Real World, a barbette is an armored structure around a gun platform on a warship. Unlike a turret; which revolves with the gun carried inside it, a barbette doesn't move. Instead the gun moves within it. The use of barbettes waned in the 20th Century.
If you look carefully at the various pre-dreadnought battleships of ~1890 to 1905 and the early dreadnoughts that replaced them, you'll see a line of relatively small caliber guns (5 inch or smaller) arranged along the upper hull amidships. Later on, these smaller guns were either placed in turrets of their own or completely unarmored aside from splinter shields.
In a stretch, the Confederate ironclads of the American Civil War can be said to carry their guns in a barbette, although their armor scheme is more properly known as a casemate.
In Traveller, a barbette is a heavy turret of sorts. It exists between the normal 1 to 3 dTon Traveller turrets and the 50 - 100 dTon weapon bays. In CT's HG2 at TL14 for instance, a PAW - which had only been available in spinal mounts or bays - becomes available in a 5 dTon barbette form. IIRC, MT introduced other barbette weaponry like meson guns and repulsors.
Sincerely,
Larsen
