 With few exceptions, even the largest siege weapons had become mobile by road or rail by the start of World War I, and evolution after that point tended to be towards smaller weapons with increased mobility.Even the German super-heavy guns in World War II were rail or caterpillar-track mobile.In British use, a "field gun" was anything up to around 4.5 inches in calibre, larger guns were "medium" and the largest guns were "heavy".