In 1943, German engineer August Cönders, of Röchling Stahlwerk AG, proposed an electrically initiated multiple-charge weapon. Thanks to the success of Cönders's other projects, including the "Röchling shell", major figures in the Nazi establishment took notice of him, most importantly Albert Speer, the Minister of Munitions. Cönders was ordered to produce a prototype of the Hochdruckpumpe ("high-pressure-pump") and duly constructed one in 20 mm calibre, which proved satisfactory. At this point, Adolf Hitler, who had been following the project with interest, took a hand and decided that a battery of 50 full-size guns would be sited in northern France for bombarding London. Cönders had constructed a full-calibre gun at the Hillersleben proving ground near Magdeburg, but by the end of 1943 he had encountered severe problems both in putting the gun's basic principle into operation and in producing a feasible design for the shells it was to fire. Even when everything worked, the muzzle velocity was just over 1,000 metres per second (3,300 ft/s), which was nowhere near what had been promised. Nonetheless, plans were proposed to build a single full-size gun with a 150-metre (490 ft) barrel at Misdroy on the Baltic island of Wolin, near Peenemünde, while construction at the Mimoyecques site in France (which had already been attacked by the USAAF and the RAF) went ahead. By March 1944, with no good news from Misdroy, the Heereswaffenamt (Weapon Procurement Office) took control of the project, and Cönders became one of the engineers working on the three chief problems: projectile design, obturation, and ignition of the secondary charges. Six different companies, including Krupp and Skoda, produced satisfactory designs for projectiles. Obturation problems were solved by placing a sealing piston between the projectile and the initial propellant charge, which in turn prevented the flash from the charge from getting ahead of the projectile and solved the problem of controlling the initiation of the secondary charges. By the end of May 1944, there were four designs for the 150-mm finned projectile, one manufactured by Fasterstoff (designed by Füstenberg), and three others by Röchling (Cönders), Bochumer (Verein-Haack), and Witkowitz (Athem). Trials were held at Misdroy from May 20–24, 1944 with ranges of up to 88 km (55 mi) being attained. On July 4, 1944, the Misdroy gun was test-fired with 8 rounds (one of the 1.8 m (5.9 ft) long shells travelled 93 km (58 mi)). The gun burst during the testing, putting an end to the tests. Following Hitler's decision that HDP guns be sited in northern France to bombard London, the task of finding a suitable site for the HDP batteries was given to Major Bock of Festung Pioneer-Stab 27, the fortification regiment of LVII Corps, Fifteenth Army, at the time based in the Dieppe area. 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. A suitable site was selected at 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 site was 8 kilometres (5.0 mi) from the sea and 165 kilometres (103 mi) from London. Codenamed Wiese (meadow) and Bauvorhaben 711 (Construction Project 711), Organisation Todt began construction in September 1943 with the building of railway lines to support the work, and began to excavate the gun shafts in October. 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 eastern complex consisted of five drifts angled at 50 degrees reaching 105 metres (344 ft) below the hilltop. The five drifts exited the hilltop through a concrete slab 30 metres (98 ft) wide and 5.5 metres (18 ft) thick. Large steel plates protected the five openings and each drift had a special armoured door. Extensive tunnels and elevator shafts supported the guns, and had the site become operational about 1,000 troops from Artillerie Abteilung 705 and supporting units would have been deployed at Mimoyecques. Artillerie Abteilung 705 had been organised in January 1944 under Oberstleutnant Georg Borttscheller to operate the Wiese gun complex. 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. Following a failure at the Misdroy proving ground in April 1944 after only 25 rounds had been fired, 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 (12,000 lb) "Tallboy" deep-penetration bombs. (Wiki). Although the bigger fixed guns were put out of operation, two shorter barreled guns were used to hit Luxembourg in 1945- only 145 rounds were fired before the Germans had to move and disassemble those guns. Later the parts were taken to the USA for testing...

http://www.historylearningsite.co.uk/fileadmin/historyLearningSite/v3.jpg http://www.militaryimages.net/photopost/data/585/v3.jpg http://www.astronautix.com/graphics/v/v3proj.jpg