Variants

• Gerät 040: original model, armed with a short 600 millimetres (24 in) calibre barrel;
• Gerät 041: later model, armed with a long (L/11.55) 540 millimetres (21 in) calibre barrel.

Long-range transport and preparation procedures

The weapon was moved long distances via rail on a variant of a Schnabel car; the whole chassis was hung between two huge pedestal-mounted swiveling arms fixed to five-axle bogies.^[7] When it reached its destination, the weapon was detached from railcars' supporting arms and driven to its intended firing location. The chassis of the ordnance was then lowered to the ground by "raising" the continuous track units' roadwheels by rotating their torsion-bar-sprung suspension arms to drop the chassis close to the ground to distribute the recoil forces more evenly in preparation for firing - the roadwheels for each trackset were also similarly "raised" to allow the tracks to clear the rails while being transported by rail, between the pair of Schnabel railcars. The Karl-Gerät proved to have no problems moving over normal soil, but under no circumstances was it allowed to make turns on soft soil lest it throw a track. The chassis had to be backed into position to fire, which expedited movement to a new position, but the land to be used for the ordnance's firing position had to be precisely leveled and the approach route prepared ahead of time to fill in soft spots and any ditches, etc.^[8] The mortar itself could only be loaded at zero elevation, so it had to be re-aimed between every shot.^[7]

1941

On 3 January 1941 Heavy Battery (schwere Batterie) 833 was created at the Bergen training ground and ordered to be combat ready by 15 February 1941. On 2 April 1941 it was expanded into Heavy Artillery Battalion (schwere Artillerie Bataillon) 833. The original Batterie 833 was redesignated as the first battery of the new battalion and a new second battery was formed, each battery having two howitzers, with orders to be combat ready by 1 May 1941 in preparation for Operation Barbarossa. Initially a single battery was to be deployed against the Soviet fortress at Brest-Litovsk, but that was changed by 14 May 1941 when the other battery was ordered to attack the Soviet border fortifications near Lviv. The first battery was assigned to IV Army Corps of 17th Army of Army Group South near Lviv while the second battery was ordered to support the attack by the 4th Army of Army Group Center against the Brest Fortress. The batteries were issued 60 and 36 rounds respectively.^[12]

Little is known of First Battery's operations except that IV Army Corps reported on 23 June that the battery was no longer needed and was no longer operational due to technical deficiencies. Second Battery's weapons had some assembly problems, issues with the electrical firing mechanism and non-standard ammunition, not surprising for the Karl-Gerät's combat debut, but managed to fire 31 of their 36 rounds by 24 June. It was ordered home that day by Army Group Center where the battalion was ordered to reform with eight 21 cm Mörser 18 howitzers on 6 August 1941.^[13]

1942

In preparation for the attack on Sevastopol scheduled for the early summer Heavy Artillery Battalion 833 was ordered to form a Karl-Batterie with three weapons on 18 February 1942, two of which were "Thor" and "Odin". Camouflaged firing positions 15 metres (49 ft) long, 10 metres (33 ft) wide and 3 metres (9.8 ft) deep had to be dug for each howitzer to minimize Soviet counter-fire before they could move into position. On 20 May 1942 the 11th Army reported all three Karl-Geräte were at the front with a total of 72 heavy and 50 light concrete-piercing shells. LIV Army Corps reported that 19 heavy shells were fired between 2 and 6 June, 54 on 7 June and all 50 light shells between 8 and 13 June. More shells (29 heavy and 50 light) were shipped to the battery before the end of the month. All 50 light shells were fired on 30 June and 25 heavy shells the following day. Many of these shells were fired at the two 305 millimetres (12.0 in) twin-gun armored turrets of the Maxim Gorkii coastal defense battery,^[14] although shells fired at the turrets had little effect other than to jam one of the turrets and possibly knock out electrical power to the turrets, both of which were repaired without too much trouble. They did rather more damage to the concrete structure supporting the turrets as well as the command center located some 600 meters away (called the Bastion by the Germans). On 19 July 1942 the battery was ordered to ship their weapons to Hillersleben for refurbishment. One dud shell was recovered by the Soviets and flown to Moscow for evaluation.^[15]

On 7 July 1942 Heavy Artillery Battalion 833 was ordered to form another battery with one or two Karl-Geräte. This was done by 15 August as schwere Batterie 628 (Karl) with two weapons, although sufficient personnel to man three guns was to be furnished by Heavy Artillery Battalion 833. On 22 July the Army High Command (Oberkommando des Heeres (OKH)) issued an order to send the battery to Army Group North to support its planned offensive, Operation George (Unternehmen Georg), against Leningrad. The order for Georg, dated 22 August, specified Battery 628 with three guns, presumably with two operational guns and one in reserve was to participate. Army Group North reported the battery's arrival on 1 and 2 September 1942, but the Soviets preempted Georg with heavy attacks against the German forces besieging Leningrad so the Karl-Geräte didn't get into action.^[16]

On 18 October OKH ordered the 11th Army to transfer the battery as soon as possible to Leipzig, but the 11th Army asked to retain it to use in a new version of Georg to begin later that month. Georg was again postponed in late October, and later canceled. A new attack, code-named Feuerzauber (Fire Magic), was planned in which the battery was to participate, but it too was canceled after the Soviet encirclement of the German forces attacking Stalingrad. OKH finally recalled the battery on 4 December 1942 when it was clear that it had no mission.^[16]

1943

OKH issued orders to create a Heavy Artillery Battalion (Karl) on 4 May 1943 using the vehicles and equipment of Heavy Artillery Battery 628. This became the first battery of the new battalion while the other battery was raised from scratch on 15 May as was the battalion headquarters. Each battery had two Karl-Geräte plus one in reserve. 18th Army of Army Group North had plans to use one Karl-Gerät against the Oranienbaum Bridgehead west of Leningrad during the summer of 1943, but the battalion was ordered to return the Karl-Geräte to Leipzig on 8 August. Just like the 833rd the battalion was ordered to reform with eight 21 cm Mörser 18 howitzers on 29 August 1943 with effect by 10 September. A Unit for Karl-Geräte (Kommando für Karl-Geräte) was formed on that same date as caretakers for the weapons. This was redesignated Unit for Special-Equipment of Heavy Artillery Battalion (motorized) (Kommando für Sonder-Gerät des schwere Artillerie-Abteilung (mot.)) 628 on 2 June 1944.^[16]

1944

600 mm Karl-Gerät "Ziu" in front of general Józef Sowiński's statue in Warsaw, August 1944

The Prudential building in Warsaw being hit by 2-ton mortar shell from a Karl-Gerät. The picture was taken by Polish soldier Sylwester Braun "Kris" on 28 August 1944.

On 13 August 1944 a battery was ordered to be created immediately with one 54 cm Karl-Gerät and sent to the 9th Army to help it suppress the Warsaw Uprising. The next day the Kommando für Sonder-Geräte formed the Army Artillery Battery (Static) 638 Heeres-Artillerie Batterie (bodenständige) with 60 cm Karl-Gerät Nr. VI "Ziu" since no 54 cm weapon was available and a firing table had not yet been computed. It arrived at the Warsaw West railway station at 07:00 on 17 August 1944; the ammunition train arrived the following morning.^[17]

The mortar "Ziu" was settled in the Sowiński Park near the statue of general Józef Sowiński in Wola district.^[18] The 600mm caliber mortar shells were designed to destroy bunkers. Often they did not explode when they hit buildings or soft ground in Warsaw. On 18 August 1944 a shell fired from mortar "Ziu" hit the building at Moniuszki 10 street, where the famous Warsaw restaurant "Adria" was located. The shell broke through several stories of the building, roof and floor of the restaurant and finally stopped in the basement, but it did not explode. Sappers of Polish Home Army disarmed the dud shell and removed the explosive charge. The explosive was used to produce hand grenades (Filipinkas) for Polish soldiers. The empty shell was left in the basement. In the mid-1960s during removal of ruins of the restaurant the shell was found. It was moved to the Polish Army Museum in Warsaw, and is a part of the museum's outdoor exhibition.^[19]

Warsaw Uprising: Sappers disarming a dud 600 mm mortar shell, which crashed through several stories and stopped in the basement of "Adria" townhouse at Moniuszki 10 street. From left: ppor. (second lieutenant) Stanisław Dutkiewicz "Pillert" i st. sierż. (master sergeant) Józef Gorzkowski "Szczepko".^[20]

On 30 August 1944, another dud shell from mortar "Ziu" was found in the basement of Prudential building located at Napoleon Square 9 (Plac Napoleona 9, now Plac Powstańców Warszawy). This shell was also disarmed by sappers of Polish Home Army.^[9][21]

After World War II a few more 600mm dud shells were found in Warsaw.^[18] The most recent one was found on 28 August 2012 about 10m below surface of the ground in the neighborhood of Plac Powstańców Warszawy during construction of the second metro line in Warsaw.^[22][23]

On 24 August 1944 OKH noted that it had been very successful in combat and ordered another Karl-Gerät sent to Warsaw. A second battery, numbered 428, was formed two days later by the Kommando für Sonder-Geräte, but it did not arrive at the Warsaw West railway station until 12:57 on 7 September 1944. A third Karl-Gerät 040 was shipped to Warsaw on 10 September and incorporated into Battery 428. "Ziu" needed repairs and was shipped on 22 September back to Jüterbog. At some point a fourth Karl-Gerät was shipped to Warsaw as it was reported as operational on 25 September.^[17]

1945

Little is known of their activities during the battle, but Gerät Nr. II was damaged en route by a bomb and was back in Jüterbog on 6 January 1945. It was repaired using parts from Karl Nr. I by 3 February. Gerät Nr. IV was damaged by a number of penetrating hits and arrived in Jüterbog on 31 January. Gerät Nr. VI was still at the front on 19 January. In the meantime Nr. V was modified to use the 54 cm howitzer and had its engine rebuilt.^[25]

Battery 628 was initially ordered to the Vistula Front on 7 March, but this was countermanded on 11 March when it was rerouted towards Remagen, where the Allies had recently captured the Ludendorff Bridge. Karl-Batterie 638 arrived on 20 March, but after only 14 rounds that missed all of the bridges and only damaged a handful of random houses, the weapon had to be moved to the rear for repairs.^[26] Batterie 428 was also ordered towards Remagen on 11 March, but was rerouted to the 1st Army sector instead.^[27][28]

A status report dated 22 March showed Geräte Nr. I and IV were still at Jüterbog, with Nr. I configured as a 040 and Nr. IV to be completed as a 041. Geräte Nr. II and V had left for the front on 11 and 10 March respectively. Gerät Nr. VI was returning to Jüterbog with engine damage. Gerät Nr. III had been virtually destroyed and only the gun breech was salvageable. Gerät Nr. VII needed parts and wasn't going to be made operational anytime soon.^[28]

Battery 628 was ordered disbanded on 11 April and its personnel incorporated into Battery 428, although it's unlikely that either unit actually saw action except in local defense.^[28]