What was the largest-calibre rifled weapon ever used in combat?

Schwerer Gustav was a German 80 cm (31.5 in.) railway gun. It was the largest-calibre rifled weapon ever used in combat and, in terms of overall weight, the heaviest mobile artillery piece ever built. It fired the heaviest shells of any artillery piece. It is surpassed in calibre only by the unused British Mallet's Mortar and the American Little David bomb-testing mortar (both 36 inch; 914 mm).

In 1934, the German Army High Command (OKH) commissioned Krupp of Essen to design a gun to destroy the forts of the French Maginot Line that were nearing completion. The gun's shells had to punch through seven metres of reinforced concrete or one full metre of steel armour plate, from beyond the range of French artillery. Krupp engineer Erich Müller calculated that the task would require a weapon with a calibre of around 80 cm, firing a projectile weighing 7 tonnes from a barrel 30 metres long. The weapon would have a weight of over 1000 tonnes. The size and weight meant that to be at all movable it would need to be supported on twin sets of railway tracks. In common with smaller railway guns, the only barrel movement on the mount itself would be elevation, traverse being managed by moving the weapon along a curved section of railway line.