Schwerer Gustav
The Schwerer Gustav was the largest calibre rifled weapon in the history of artillery to see actual combat. It fired the heaviest shells of any artillery piece. A few other huge guns were never fired in anger: the French Monster Mortar (36 French inches; 975mm), the  British Mallet's Mortar (36 inch; 914 mm) and the American Little David mortar (36 inch; 910 mm).