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The Karl-Gerat (040/041) (or "Karl device" in German) was a self-propelled siege howitzer designed and built by Rheinmetall in World War 2. The howitzer became the largest self-propelled gun ever built and used in combat. Despite its size and available firepower, the system had all the inherent weaknesses of all previous large-caliber cannon platforms to which its combat usefulness was questionable.

Hitler was always ready to invest resources into new weapon systems if they had great destructive potential and could terrorize his enemies. The V-2 rocket and the heavy water project that would have lead to an Atomic bomb were two other such projects. In 1936, the munitions designer and builder Rheinmetall made a proposal to the German High Command for a super heavy howitzer capable of destroying the defenses of the Maginot Line defending France from German invasion. On paper this was no easy feat for the defensive line stretching along the French eastern border between Germany and Italy was a formidable one and constructed based on French experiences garnered in World War 1.

This defensive line ranged between 20 to 25 kilometers deep and was built with a complex system of strong points including 34 turrets of 75-mm gun emplacements, concrete fortifications, 19 turrets of 50-mm motors, barricades, 61 machine gun turrets providing overlapping and concentrated fire, 17 turrets of 135-mm guns and the like. Hitler knew that war was coming and his first strike would be to the West. The order was given to build the 040 before Germany had officially committed to war but Hitler went ahead and attacked early when world events seemingly gave him the advantage. Thusly, the new howitzers were not made ready in time for the German Blitzkrieg against Poland and France. As it turned out, the French Army was not as formidable as their Maginot Line and the fortifications were quickly dispatched by standard German armor and infantry. Patton was once quoted as saying "If mountains and oceans can be overcome then anything man has made can be also be overcome". How true it proved for these French defenders.

Seven Karl-Gerat howitzers were ultimately manufactured. In charge of the weapons program was General Karl Becker, a German weapons engineer and artillery general. He promoted close ties between the German military and scientists for weapons development. Due to his leadership in the development process of this particular gun, his men named the weapon after him. The Rheinmetall factory delivered the six production vehicles (the seventh was held in reserve for testing) from November 1940 to August 1941. All six of these guns saw action across the war's many fronts.

During the development phase the initial concept was that the weapon would be transported by several tracked vehicles and assembled on site. These road trials proved that the process required an extended setup time and needed more troops than was originally planned. These requirements worked against the usefulness of the Karl and led the design team to change the Karl from a towed-weapon to a self-propelled weapon in 1937. This change reduced the time needed to deliver the weapon to the target site, setup and fire the gun while using less manpower. Extensive driving and setup trials took place in 1938 and 1939 using a prototype scale model of the weapon. The road tests were carried out over different soil types to see if the weight of the super-heavy vehicle could traverse the terrain as expected. Existing tank chassis were modified for such projects: for support vehicles they chose the Panzer IV and modified twenty two Ausf. D, E and F models as such. The turrets were removed and the superstructure was retooled for each transport to carry four 3,700 lb shells and a 35-ton crane needed to load and transfer the ammunition. Based on the mission, each Karl needed two or three of these support vehicles to carry their applicable heavy munitions.

The Karl operated form an opened flat wedge breech. The mechanical loaded would push the shell into the barrel. The entire gun was affixed to a thick massive baseplate for the tremendous recoil. The muzzle velocity was measured at 6,950 meters per ton. The loading tray with the rammer was mounted onto the rear of the gun carriage. Due to the difference in diameters, the loading tray for the 60-cm shells could not be used for the 54-cm shells. A large gap was left between the carriage and the engine compartment at the front of the vehicle so that the entire carriage could recoil fluidly.

At 124 m/ton and 273,374lbs, the Karl vehicle needed a large engine. General Becker decided to used the Daimler-Benz MB 503 or the MB 507 C series engines - both were 12-cylinder liquid-cooled gasoline engines. These powerplants provided a maximum speed of 6.2 miles per hour (10 km/h). When target distances were far enough away making the 10 mph vehicle impracticable, the Karl was disassembled using a 35-ton mobile crane mounted on the support vehicles and loaded onto seven lighter four-axle trailers. The weapons chassis was loaded onto a six-axle Culemeyer lowboy trailer. The Gerat 041 introduced a new suspension system tied to 11 steel road wheels to a side.

To move the weapon long distances via the European rail network, a variant of a Schnabel car was produced. This special car allowed the whole chassis of a large vehicle to be hung between two huge pedestal-mounted swiveling arms fixed to a five-axle flat car. When it reached its destination, the weapon was detached from its supporting arms, driven to its intended firing location and set up as normal. At the firing location, the chassis was lowered to the ground to distribute the recoil forces when fired. The Karl-Gerat proved to have no problems moving over normal soil but under no circumstances was it allowed to make turns on soft soil - the reason being that soil types might allow it to throw a track (that is, have the tank tread fly off of its wheels). A separate crew arrived at the firing location before the Karl could be fired. The approach route had to be prepared ahead of time to fill in soft spots along the path and to have any ditches leveled. The firing position had to be precisely leveled because the chassis had to be backed into position to fire. The weapon could only be loaded at zero elevation and it had to be re-aimed after every shot.

Firing trials took place in June 1939 and met with the anticipated result. The destructive power of the 60-cm concrete-piercing shell made a crater up to 15-m (49-ft) wide and 5-m (16-ft) deep. However, immediate discussions ensued due to concerns of the range of the weapon. The 60-cm shell had a reported range of up to 4,720 yards (2.68 mi). This distance left the howitzer within range of smaller and more accurate enemy artillery pieces. There was also concern for the safety crew working with their many vehicles and operating in a protracted setup time before firing could commence. The crew and all applicable equipment would be susceptible to enemy fire from land or air. In short, the 040 was not your typical hit-and-run weapon.

The development section at Rheinmetall began to design a longer-range barrel to help alleviate the short-range worries and, by May of 1942, a smaller- and longer-range weapon system was ordered and ultimately delivered with a 54-cm barrel. The 041 had a range of 11,000 yards, 10,500 meters (6.25 mi). Hitler was told the weapon would be operational in March 1943 and the first three 54-cm Gerat 041s would be delivered to field units between June and August 1943. Only three 54-cm barrels were actually completed by war' end and they were mounted on Adam, Odin and Loki. In case of malfunction the smaller longer-range 040 barrel could be used on any of the other weapons with minimal conversion.

In the summer of 1941, part of the Operation Barbarossa (the German invasion of the Soviet Union) plan was to take the Brest fortress in Russia. The layout of the Brest fortifications was such that its defenses were overlapped via a star-shaped pattern. For the invading force, these fortifications could quite difficult to overcome. This fort was built by military engineers so that the walls were embedded into ditches fronted by earth slopes, defending the fortress from attacks through direct fire. The walls were topped with earth mounds that absorbed (and largely dissipated) the energy of plunging fire. Fortresses of this type were more or less a medieval incarnation and occupied key positions in warfare for centuries in Europe and Asia. The ditches and walls focused attacking troops into concentrated "kill zones" where cannon, machine gun or small arms fire could inflict maximum concentrated carnage on the assaulting troops. The core of the fortress, named the Citadel, was a red brick two-story round building that was 1.8 km long with 500 rooms, 4 gates and 4 semi-towers that could house approximately 12,000 infantrymen. Furthermore, the Citadel was located on an island formed by the Western Bug River and the two branches of the Mukhavets River. The Citadel was surrounded by 3 bridgeheads and fortified by earthworks that were 10 meters high and surrounded in redbrick casemates. By this time, the German Army had made little headway against the Soviet structure and taking heavy casulaties in the process. As such, the Karl was called up.©MilitaryFactory.com

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.©MilitaryFactory.com

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