The treaty repeatedly returns to 6.1-inch calibre because it marked the boundary of cruiser armament. Royal Navy destroyers and sloops normally carried much smaller 4.7-inch and four-inch guns. The restriction therefore bore hardest on powers seeking maximum gun power in destroyer-sized hulls. Later interest in 5.9-inch weapons shows how designers could search for room immediately below the treaty threshold.
Notable Point · 4:53-6:00
The six-inch threshold and small-cruiser pressure
Six-inch guns marked the point at which a ship became a dangerous cruiser, pressuring smaller navies not to turn destroyers into miniature cruisers.