The V-3 or "High Pressure Pump" (HDP for short) was a German supergun which used multiple propellant charges placed along the barrel's length and timed to fire as soon as the projectile passed them, to provide an additional boost. The 140 m long cannon was capable of delivering a 140 kg shell over a 165 km range. Construction began of a bunker for the cannons in September 1943 at Mimoyecques, France. The site was damaged by Allied bombing before it could be put into operation and was finally occupied by the British at the end of August 1944. Two short-length (45 m long) V-3's were built at Antwerp and Luxembourg in support of the Ardennes offensive in December 1944. These were found to be unreliable and only a few shots were fired without known effect.

The weapon was planned to be used to bombard London from two large bunkers in the Pas-de-Calais region of northern France, but they were rendered unusable by Allied bombing raids before completion. Two similar guns were used to bombard Luxembourg from December 1944 to February 1945. The supergun used two batteries to crush London under a barrage of hundreds of shells per hour, shells of 140 kilograms (310 lb) with an explosive charge of 25 kilograms (55 lb).

Following Hitler's decision that HDP guns be sited in northern France to bombard London, a study, in early 1943, concluded that a hill with a rock core would be most suitable, as the gun tubes could be placed in drifts (inclined tunnels) and support equipment and supplies located in adjacent tunnels. The guns would not be movable, and would be permanently aimed at London. 

V-3 Prototype  [Via]

The site selected was a limestone hill, about 5 kilometres (3.1 mi) north of the Hidrequent quarries, near Mimoyecques in the Pas-de-Calais region of northern France behind Cap Gris Nez, very close to the French end of the present day Channel tunnel, where V-1 and V-2 launch sites were already under construction. The initial layout comprised two parallel facilities approximately 1,000 metres (3,300 ft) apart, each with five drifts which were to hold a stacked cluster of five HDP gun tubes, for a total of 50 guns. Both facilities were served by an underground railway tunnel and underground ammunition storage galleries.

The plans were to have the first battery of five gun tubes ready for March 1944, and the full complex of 25 gun tubes by 1 October 1944. the project was further cut back, from five drifts to three even though work had begun on some of the other drifts. The site was finally put out of commission on 6 July 1944, when bombers of RAF Bomber Command's 617 Squadron (the famous "Dambusters") attacked using 5,400-kilogram (11,900 lb) "Tallboy" deep-penetration bombs.

V-3 Installation [Via]

The project eventually came under the control of the SS, whose ordered the project to be ready for action in late 1944. A battery of two shorter guns approximately 50 metres (160 ft) long with 12 sidechambers were constructed and placed in the hands of the army artillery unit Artillerie Abteilung 705. These were sited in a wooded ravine of the Ruwer River at Lampaden about 13 kilometres (8.1 mi) southeast of Trier in Germany.

The two guns were aimed west, resting on 13 steel support structures on solid wooden bases on a 34 degree slope. The city of Luxembourg (which had been liberated in September 1944) was at a range of about 43 kilometres (27 mi) and was designated Target No. 305

By the time the Ardennes offensive began on 16 December 1944, Kammler received orders from OB West (German Army Command in the West) to begin firing at the end of the month and on 30 December 1944 the first gun tube was ready for action. Two warm up rounds were initially fired, followed by five high-explosive shells which were fired in sequence, attended by Kammler. The muzzle velocity was approximately 935 metres per second (3,070 ft/s).

V-3 Projectile [Via]

The second gun tube was brought into operation on 11 January 1945 and in total some 183 rounds were fired until 22 February 1945, with 44 confirmed hits in the urban area. The guns were not particularly effective; from the 142 rounds that struck Luxembourg, total casualties were 10 dead and 35 wounded.

One gun was dismantled on 15 February, and firing ceased on 22 February, when US Army units had advanced to within 3 kilometres (1.9 mi) of the Lampaden site.

A second battery of guns began to be deployed in January 1945 at Buhl, aimed at Belfort in support of the Operation Nordwind offensive. One gun was erected before the failure of the Nordwind offensive put the site at risk, and the equipment was removed before firing could begin.

V-3 Gun [Via]

By the end of August the Germans completely abandoned the complex in the face of the advancing British forces. The British dynamited the Mimoyecques complex on 9 May 1945.