"The idea of producing a German 128-mm (5.04-in) anti-aircraft gun was first mooted in 1936 when Rheinmetall was requested to produce a design known then as the Gerat 40. Progress on this design was not placed at a very high priority, so it was not until 1940 that the first prototype was ready. At that time it was intended that the Gerat 40 would be a weapon for the field army, but when the military saw the size and bulk of the prototype they decided that the weapon would be produced for static use only. The weapon was ordered into production as the 12.8-cm Flak 40. By that time plans had already been made for a production-line mobile version, so the first six were produced on mobile carriages.
http://henk.fox3000.com/Attack-resin.htm - mobile and zwilling version by Attack
The Flak 40 was so large that it proved impossible to carry the gun in one load, other than very short distances, so a two-load system was initially employed, even this proved to be too cumbersome, and was later revised to a single load once again.
Later versions were produced for static use only, and such was the overall performance of the Flak 40 that it was carefully emplaced around some of the main production and population centres such as Berlin and Vienna, Special Flak towers were built in some locations to make best use of these guns, and there was also a special railcar version to provide the guns with some sort of mobility.
Production of the static version began in 1942, but it was a costly and complex gun so by January 1945 there were only 570 in service, all of them based inside the borders of the Reich, Soon after full-scale production began, the Flak 40 was joined by a twin version of the same gun known as the 12.8-cm Flakzwilling 40.
http://www.svsm.org/gallery/128mm_flak40 - walkaround
This consisted of two 12.8-cm (5.04-in) Flak guns mounted side-by-side on the same mounting and provided with ‘mirror’ loading arrangements. These powerful gun combinations were used only on special Flak towers around the main centres of population within the Reich, and were so costly and difficult to produce that there were never many of them; even by February 1945 there were only 33 in service. The Flakzwilling (Zwilling, or twin) was introduced as it was realized that ever heavier anti-aircraft guns would be needed to counter the increasing performance of Allied bombers, and despite strenuous efforts to develop guns with calibres of 150 mm (5.9 in) and even 240mm (9.45 in), none got past the prototype stage at best and some failed to get even that far, Thus the twin arrangement of the Flakzwilling 40 was an attempt to produce at least some form of increased firepower to counter the Allied heavy bombers, and in the event it turned out to be an excellent anti-aircraft weapon. As the war ended the original mobile Flak 40s were still in use, many more were in use on special Flak trains. A new 12.8-cm Flak 45 gun was under development as the war ended, and this would have been an even more powerful weapon than the original. Only a single prototype was completed." - Jim, war44.com
12.8-cm Flak 40
Calibre: 128 mm (5,04 in)
Weight: travelling (mobile) 27000 kg (59,524 kg), firing (mobile) 17000 kg (37,478 lb), and firing (static) 13000 kg (28,660 lb)
Dimensions: length overall 15 m (49 ft 2.6 in); height 3.965 m (13 ft); length of barrel 7.835 m (25 ft 8.5 in); length of rifling 6.478 m (21ft 3 in)
Maximum effective ceiling: 14800 m (48,555ft)
Shell weight: 26 kg (57.3 lb)
Muzzle velocity: 880 m (2,887 ft) per second
- Waffen-Arsenal - So15 - Deutsche schwere Flak 36-56