42 cm Gamma Mörser

With a maximum range of 14,000 m (46,000 ft) and a high degree of accuracy, the Gamma-Gerät was the Imperial German Army's most powerful siege gun. The Gamma-Gerät was essentially an enlarged Beta-Gerät 09 weighing 150 metric tons (150 t), ensuring that it could only be emplaced near permanent railways in a process that took 24 hours. To emplace the Gamma-Gerät, all nearby vegetation was cleared and a rectangular 2.25 m (7.4 ft) pit was dug and then reinforced with timber and steel to form the bedding. At the same time, a rail spur was spun off the nearest permanent line to the site. Then, a 25 metric tons (25 t) rail-mounted gantry crane was unloaded and used to assemble all seven, 20–25 metric tons (20–25 t) portions of the Gamma-Gerät. Fully assembled, the gun stood 4.25 m (13.9 ft) high and 13.5 m (44 ft) long. Its standard rate of fire was one shell every seven minutes and eight shells in an hour. The fastest recorded rates of fire were 19 shells an hour for a single Gamma-Gerät and 30 shells an hour for a two-gun battery. The gun had an elevation of 43° to 66°.[10]

A 42cm projectile in 1918

German siege artillery had three types of projectiles: armour-piercing, high-explosive, and intermediate. The armour-piercing shell was designed to smash through concrete and metal armour, but was largely ineffective against reinforced concrete. High-explosive shells were fitted with two charges and could be set to have no delay, a short delay, or a long delay. If set to "no delay," then the shell burst on impact. 42cm high-explosive shell craters could be as wide as 9 m (30 ft) and as deep as 6 m (20 ft). If set to a delayed detonation, it could penetrate up to 12 m (39 ft) of earth. Finally, the intermediate, or "short shell," weighed half as much as the high-explosive shell and was fitted with a ballistic tip for range and accuracy. 42cm shells were generally 1.5 m (4.9 ft) long, weighed between 400–1,160 kg (880–2,560 lb), and were propelled via primer loaded into the gun with a brass casing. Siege artillery shells were produced in limited runs of varying quality. Beginning in early 1916, German siege guns began to suffer internal explosions due to faulty ammunition. As a result, crews were required to disembark from the gun before firing via a lanyard.[11]

With the start of World War I, all siege-gun batteries were mobilised and assigned to the Western Front. KMK Battery 2 was sent north to Namur, but could not arrive in time to participate in the Siege of Namur, while KMK Battery 1 was assigned to the 6th Army in Lorraine. As part of the 6th Army, it participated in the siege of the Fort de Manonviller from 25–27 August 1914. During the 52-hour siege the battery had technical troubles and had to stop firing. KMK Battery 1 finally arrived at the then-ongoing siege of Maubeuge in early September and joined the siege guns already present in obliterating Forts Leveau, Héronfontaine, Cerfontaine, and Sarts. On 7 September 1914, with only two forts remaining in French hands, Maubeuge's garrison surrendered.[13]

42cm shell crater at the Fort de Boussois

The German defeat at the First Battle of the Marne prevented the siege guns at Maubeuge from being sent against Paris, so they were sent back into Belgium to Antwerp.[14] The Belgian Army, which had retreated to the city on 20 August following the fall of Liège, had made attacks on the German flank on 24–25 August and 9 September. In response, the III Reserve Corps, from the 1st Army, was sent to capture the city.[15] The Corps arrived at Antwerp on 27 September, partially surrounding it and massing at its southern side. The next day, KMK Battery 2 opened bombardment against the Fort de Wavre-Sainte-Catherine, which was destroyed on 29 September by a magazine detonation caused by a 42cm shell. KMK Battery 2 then shifted its fire to the Fort de Koningshooikt, which surrendered on 2 October.[16] Beginning on 7 October and lasting the next two days, Antwerp's defenders began withdrawing from the city, which then surrendered on 10 October.[15] With the fall of Antwerp, KMK Battery 2 was attached to the 4th Army to aid in its capture of the Channel ports, and occasionally shelled Nieuport, Ypres, and Diksmuide.[17]

On 27 February 1915, KMK Battery 1, with the 8th Army, joined the ongoing attack on Osowiec Fortress. Although the fortress was made of masonry, it survived, intact, because the artillery had no spotters to guide its fire and because of effective counter-battery fire from the fortress. After five days, the siege guns were withdrawn and the fort was besieged until August. On 8 August, KMK Batteries 1 and 4 fired on Kaunas Fortress to support the XXXX Reserve Corps. While slow, the bombardment was highly effective because of outdated masonry construction, and the Germans took four forts altogether on 16–17 August. The Russians evacuated from the city and fortress the next day. The last action for Gamma-Geräts in the East was the German Invasion of Serbia. On 6 October 1915, KMK Batteries 1 and 4 opened fire on Serbian fortifications east of Belgrade to support the crossing of the 11th Army, made the next day. Battery 1 briefly fired on Smederevo Fortress, but the fortress was undamaged when it surrendered on 11 October.[18]

KMK Batteries 1 and 4 were transferred back to the Western Front in early 1916. The latter bombed Forts Douaumont and Vaux without effect while the former inflicted serious damage on a French railway viaduct east of Belfort. At the beginning of that year, all 42cm guns were assigned to the 5th Army for the upcoming Battle of Verdun.[19] The battle began on 21 February with an intense, nine-hour artillery barrage.[20] The 42cm guns were tasked with bombarding Forts Douaumont, Vaux, Souville, and Moulainville, the most modern fortresses at Verdun, to silence their guns and prevent French units from rallying at them. However, despite heavy shelling by all thirteen 42cm guns, the forts were only lightly damaged. At the same time, French counter-battery artillery and internal explosions plagued German siege guns; KMK Batteries 2, 8, and 9 lost a Gamma-Gerät each. In July, siege batteries began to be withdrawn north to the Battle of the Somme, and east to Romania. KMK Battery 4 remained at Verdun, while Battery 1 attacked Arras and Loos-en-Gohelle in June and July.[21]

In the last two years of the war, siege guns saw limited use and negligible effect on the Western and Eastern fronts. For the Spring Offensive, KMK Battery 4 was assigned to the 17th Army on the Somme. When the time for Germany's final offensive operation came in July, the gun was assigned to the 1st Army at Reims but had accomplished little in the battle. KMK Battery 4 survived the reorganization of the German Army in late 1918, but with a single gun.[22]

A single Gamma-Gerät survived World War I and its aftermath, disassembled and hidden in Krupp's Meppen facilities. The gun was reassembled in the late 1930s and was used in 1940 to bombard both the Maginot Line and the city of Liège. Later, it was deployed to Sevastopol in 1942 and then at Kronstadt, but did not fire a single shot there. The gun returned to Germany and its post-war fate is unknown.[23]