I have recently came across information about a mortar, may I reiterate a massive mortar built in 1917 by SKODA at the torková engineering works in Brno. This massive 500mm mortar was tested by Germany and Austro-Hungary at the shooting range in Steinfeld and Husovice-u Brna.
One prototype was produced in 1917
The shell weighing in at 270 kg /595 lbs.
Range was 2000m/6561ft.
It name as I understand it was the Storek Mortar System.
The photo's are of the prototype housed in the Military Technical Museum Leany of the Czech Republic
Some information I came across in a translation, hope it helps:
50cm STOREK Mortar
CALIBRE: 500 mm
GRANADE WEIGHT: 250 kg
MANUFACTURER: Austria-Hungary, Storkova strojirna, Brno
YEAR OF PRODUCTION: 1917
Skoda Plzen was not the only factory in the Czech Republic in the First World War II which engaged in the production of mortars. This production, together with the production of ammunition, was divided among the series of smaller engineering works and foundries. The most important of these was the engineering works Storkova Strojirna Brno, where a number of mortars of completely original design had been produced. Among them, there was the 5 cm mortar, which was later used by the Czechoslovak Army until 1929. The exhibited 50 cm mortar was one of the largest structures made in the Storkova Strojirna. A single prototype was made between 1917 and 1918 and tested by the Austrian and German army; however, it was never introduced into the armaments of these countries. The cartridge, weighing 250 kg, was lowered into the hinged end of the mortar by a crane.