On April 2nd, 1941, the Heavy Battery Number 833 of the new battalion was formed, each battery having two howitzers. Orders arrived to be combat-ready by May 1st, 1941, in time for Operation Barbarossa. Initially, a single battery was deployed against the Soviet fortress at Brest-Litovsk while the second battery was ordered to support the attack by the 4th Army of Army Group Center against the Brest Fortress. Being their first deployment in combat, not much is known of the Battery's operations except that IV Army Corps reported in late June that the battery itself was no longer operational due to technical firing problems. The second battery's weapons had some assembly problems along with an electrical firing mechanism issue. The size of the shells required some individual production in a few armories so each shell was made as non-standard ammunition, effectively not to precise calibrations. The Karl-Gerat's second battery's introduction to a combat situation fired 31 of their 36 rounds at the fort by June 24th. The battery was ordered home by Army Group Center where the battalion reformed with eight 21-cm Morser 18 howitzers in August of 1941. Karl did not break the fortifications as expected due to her aiming problems that effectively reduced concentrated fire and added to her already existing mechanical issues.
The Karl was used for the shelling of Sevastopol by the Germans in 1941 and into 1942. Sevastopol was under siege by the German Army and was corralled for 250 days before the city fell in July of 1942. Heavy Artillery Battalion 833 was ordered to form a Battery comprised of three Karl howitzers and support vehicles in February of 1942. Support forces for the Karl weapons traveled to Sevastopol to survey firing sites in preparation for the attack on the city to be scheduled for the early summer. The support teams dug these future secret firing positions so the battalion could quickly move into the firing positions with little lost time. Each pit for a Karl was 15 meters (49 ft) long, 10 meters (33 ft) wide and 3 meters (9.8 ft) deep and had to be dug for each howitzer to minimize Soviet artillery return fire.
On May 20th, 1942, 11th Army reported that all three Karl's were at the Sevastopol front with a total of 72 heavy and 50 light concrete-piercing shells. LIV Army Corps reported that between June 2nd and June 6th, each gun only fired 6 heavy shells apiece. The battery rate-of-fire increased with 54 shells fired between June 7th and June 8th and 50 light shells were fired on June 13th. The army shipped 79 more shells to the battery before the end of the month, and all were fired. The primary target for most of the shells were two 305-millimeter (12.0 in) armored turrets of the Maxim Gorkii coastal defense battery protecting Sevastopol. The Russians converted these naval guns to fire shells weighing from 321 to 351 kilograms (710-770lb) at a maximum range of 31,130 meters (34,040 yd). The railroad mounts had a maximum elevation of 40-degrees. German shells fired at the Russian turrets by the Karl battery had little effect except for a minor hit to one of the turrets that stopped electrical power, a problem that was quickly addressed by the Russians. The major damage was to the concrete structure supporting the turrets and the gun command center. In mid July of 1942, the Karl battery was ordered to ship its guns to Hillersleben for needed repairs due to the extended use.
In July of 1942, Heavy Artillery Battalion 833 was ordered to form yet another gun battery. This was done by mid-August as Battery 628 with their two guns. For reasons unknown, personnel to man three guns was assigned by the Heavy Artillery Battalion 833. In late July, the Army High Command (OKH) issued orders to send a battery to Army Group North to support its planned offensive - Operation George - against the Soviet city of Leningrad. The order for Operation George specified that Battery 628, having three guns, was to be sent with two guns to the front with one held in reserve.
Battery 628 arrived on September 1st, 1942 to Army Group North, however the Soviets began heavy attacks against the German forces attacking Leningrad, forcing Operation George to be cancelled and the Karl-Gerat guns held back for new orders. The orders came in October when (OKH) ordered the 11th Army to transfer the battery as soon as possible in support of the Leipzig Offensive. With plans to use the battery in a new version of Operation George later that month, the 11th Army requested to retain 628. Operation George again was canceled and replaced with a new plan code-named "Fire Magic" but this too was canceled when the Soviets surrounded the German forces attacking Stalingrad.
Karl battery's continued to be used until the end of the war on both the Eastern and Western fronts, being moved to quell hot spots though without the desired effects of the OKH. They were used to destroy large areas of the Warsaw Ghetto to try and slow down the advancing Russian infantry but destroying these many buildings did little to stop the Red Army. A battery was sent in support of the Battle of the Bulge but encountered mechanical difficulties and was strafed by an allied aircraft and damaged. Battery 628 was ordered to the west to the Remagen area and it was reported to have fired 14 rounds at the Remagen Bridgehead on March 20th, 1945 though without destroying the bridge or slowing the allied advance into Germany. Essentially, the Karl's had not been decisive in any single engagement and the value of such a massive weapon was highly questionable.
In 1945, No. II ("Eva") and No. V ("Loki") were captured by Allied forces. No. VII, the test weapon, was captured by the US Army in Hillersleben and was shipped to the Aberdeen Proving Grounds for evaluation. For whatever reason, this weapon was not saved as a museum trophy but later scrapped. No. VI ("Ziu") was captured by the Red Army in April 1945 and set on display at Kubinka, marked incorrectly as No. I ("Adam"). No. IV ("Odin") was also captured by the Red Army along with the other guns, the fate of these weapons remains unknown.
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