A Krupp K5 railway gun fires off a round, circa 1944.  It shot a 255kg shell out of it's 21m long barrel, up to a distance of 64km.

Railway Gun

A pair of German KARL mortars at Sevastopol.  These were 600mm, self propelled artillery pieces.  They fired a 2,170 kg projectile at a range of 7 miles.  They needed the assistance of a 35 ton crane for the loading process.

Is bigger better?

Monte Camino (Just south of the Gustav Line) November - December 1943: A German anti-aircraft gun crew keep watch in the snow covered hills near Monte Camino, Italy.