The international record of heavy-gun development places the 20-inch proposal in context. A substantial group of powers could produce weapons from 11 to 14 inches, but the field narrowed as calibre increased beyond that point. Fewer states built working 18-inch guns, and the number that mounted and fired them from ships was smaller again. This drop-off shows that extreme-calibre gunnery depended on unsettled metallurgy and barrel science as well as political will and factory equipment.
Notable Point · 36:10-37:25
Gun-building capability narrows sharply above 14 inches
Many powers could develop guns up to about 14 inches, but far fewer built working 18-inch weapons and fewer still deployed them for firing at sea.