The Orion class supplies the 13.5-inch example after Dreadnought's 12-inch starting point. A 14-inch bore looks only slightly larger in the comparison. The extra half-inch is nevertheless described as enough to matter, without quantifying its effect on gunnery performance.
Notable Point · 0:33-0:44
Orion's 13.5-inch guns and the half-inch step to 14 inches
The Orion class's 13.5-inch calibre is close to 14 inches, although the half-inch difference is described as enough to matter.