The 'Karl' Self-Propelled Siege Mortar
By Mike Bennighof, Ph.D.
September 2012

During the First World War, the Austro-Hungarian army’s heavy siege mortars helped the Germans destroy Belgian fortresses and advance on Paris. Once the Nazi regime began re-arming and considering future warfare, the need for heavy artillery to smash the new fortifications of the Maginot Line and similar works became apparent.

In 1937, the Army ordered a 600mm self-propelled siege mortar from Rheinmetall-Borsig, the conglomerate which built most of the German Army’s artillery. Named “Karl” for Karl Becker, the artillery general charged with program oversight, a prototype was produced in 1939 and driving trials took place in May 1940.

The “Karl” prototype vehicle.

The first production models, named Adam and Eve, were delivered in the first week of November 1940, well after the campaign in France and Belgium had ended. A third weapon, named “Thor,” arrived in February 1941 and a fourth, “Odin,” in April. The four vehicles were assigned to the 628th Heavy Artillery Battalion and grouped in two two-gun batteries for the invasion of the Soviet Union. A fifth vehicle, “Loki,” was delivered in June and a sixth, “Ziu,” in August; these operated at the siege of Sevastopol as the 833rd Heavy Artillery Battalion.

A very large vehicle, the Karl self-propelled mortar weighed 124 tons and was 11.37 meters long; a Royal Tiger tank, in contrast, weighed 68 tons and was 10.3 meters long. The Daimler-Benz MB507 diesel engine could move the huge gun at a speed of 10 kilometers per hour, though its ground pressure was enormous and it could not cross many bridges. It appears to have badly damaged most roadbeds it crossed. Before firing, the vehicle would lower itself to ground level to take recoil pressure off the tracks.

The mortar “Thor” on its special railway carriage.

Though officially “self propelled,” the guns did most of their travelling by rail. A special set of rail cars held the big mortar suspended between them. For longer road distances, the mortar would be disassembled and then put back together at its firing site using a special 35-ton crane. Once assembled, the mortar could fire one round about every 10 minutes. The two-ton shells had a very high trajectory, but a range of only 6,800 meters.

The first four mortars shelled Lvov in July 1941. The relatively short range had already concerned the designers before their first action, and in May 1942 longer 540mm barrels were ordered for all six vehicles. These would be interchangeable with the shorter, larger weapon, allowing the guns to swap range for firepower. The 540mm version had a range of about 10,400 meters, and launched a shell weighing about one and a half tons. When carrying the 600mm barrel the vehicle was known as Gerät 040, and when mounting the 540mm they were designated Gerät 041.

The guns saw relatively little action; the war would not be one with many protracted sieges. They performed well at Sevastopol in the summer of 1942, and would be transported northward for the abortive assault on Leningrad later that year. They probably saw their most sustained use during the “Warsaw Rising” of August 1944, pouring heavy shells into civilian neighborhoods in a vain effort to break the Polish Home Army’s will through terror and murder.

Weapon of mass terror. “Ziu” fires on Warsaw, 21 August 1944.

The Red Army took one vehicle (“Adam”) back to the Kubinka armor museum as a trophy of war, while the Americans hauled off Eve and Loki. The big mortars arrived on the scene too late for the campaign for which they had been designed, but their huge size made them favorites of Nazi propaganda photographers. They proved difficult to use in practice thanks to their very short range; they could only be deployed behind a very stable front and became a tempting target for counter-battery fire and raiding parties.

We provided two of them in our Panzer Grenadier: Edelweiss supplement, one of each version on special double-sized game pieces. We did not, however, include rules for their use. Here are a few.

Special Rules for 'Karl'

1. The Karl mortar must wait one turn between moving and beginning its firing sequence (the crew is preparing the weapon). It may not come under enemy fire of any type or undertake any game action.

2. The Karl mortar’s fire must be pre-plotted in the preceding turn: the German player writes down the target hex (do not reveal this to the opposing player). During a plotting turn, the Karl mortar may neither move nor fire, nor come under enemy fire of any type or undertake any game action. If any of these occur, the Karl mortar may not fire in the next turn. As with all bombardment fire, the target hex must be spotted during the plotting turn.

3. Once fire has been plotted, the Karl mortar may fire on the same target hex each turn. For example, the Karl mortar may move on Turn Three and Four, prepare to fire on Turn Five, plot its fire on Turn Six and fire on Turn Seven. It may fire once each turn thereafter as long as it fires on the same target hex. If the German player wishes to shift to another target hex, the Karl mortar must spend one turn plotting the new target.

4. The Karl mortar may fire only on its pre-plotted target hex, and must carry out its fire mission (if a German unit moves into the hex in the interim, it’s a very big mistake for the Germans). The German player receives no friendly fire (9.52) modifier for the Karl mortar’s fire; it strikes German units in an adjacent hex on a result of 1 or 2.

Click here to buy Edelweiss, and your very own Karl mortar counters!