7 . 5 cm L / 45 M / 32 anti aircraft gun The 7 . 5 cm L / 45 M / 32 anti - aircraft gun was designed and manufactured in Norway in the 1930s . The mount was an unusual design , having a platform with three outriggers instead of the usual four . Its main use was for positional air defence of important cities and installations . Background and design . Around 1930 it became clear that a more modern anti - aircraft gun than the M / 16 , which had been designed in the later half of World War I , was needed . A number of 76 mm anti - aircraft guns had been bought from Bofors in 1928 , but were no more modern than the M / 16 . In 1932 a new and innovative anti - aircraft gun was designed . Partly based on the M / 16 , it was mounted on a low - slung trailer with three outriggers . Two of them folded together for transportation , and two rubber wheels could be mounted . The outriggers could be adjusted in elevation to a limited extent to help place the mount in a horizontal position . The mount allowed for a 360° traverse and an elevation between 0° and 85° . The gun had a muzzle brake to reduce recoil , a hydro - pneumatic recoil cylinder and a gas cylinder to aid the forward movement of the barrel at high elevations . In addition the mount had two heavy springs to counterbalance the barrel , as the entire barrel was mounted ahead of the pivots . It ( the barrel ) , was 45 calibre s long and had 28 grooves ( rifling ) twisting to the right - like the M / 16 ' s barrel . The breech block was also similar to the M / 16 : a semi - automatic horizontal wedge that the gunner had to open manually to remove the case from the previous shot , but which closed automatically on insertion of a new round . Like late models of the M / 16 , the M / 32 was equipped to receive gun - laying information from a central sight , and also had a device for automatically adjusting the timing of the fuses on the shells . Service history and fate . The M / 32 , first issued to units of the Norwegian Army in 1936 , was well liked . First used at the anti - aircraft school based at Stavern , guns were also issued to the units responsible for defending Oslo and Raufoss . It remained in Norwegian service until the Germans invaded Norway on 9 April 1940 . At Raufoss , a total of four M / 32s were installed in two batteries . After observing several aircraft out of the guns ' range , one of the units was sent to Elverum to protect the Parliament and the King . These two guns were destroyed and abandoned when the battery retreated . The remaining two - gun unit at Raufoss fired at aircraft in the evening of 9 April , with no visible result . After several days , the battery was moved to Dombås , where one of the guns was damaged while being set up . At Dombås the last M / 32 was used to engage German paratroopers on the ground , leading to the surrender of the German force , as well as firing at German bombers with unknown results . In Oslo , the M / 32 was installed in four batteries of three guns each . These units were well equipped , by contemporary Norwegian standards , with central sights , range and height finders . When the Germans attacked Oslo , the batteries all took part in the defensive battle . In spite of a lack of ammunition , unreliable fuses , electrical batteries that discharged too quickly , central sights that fell apart due to the shock waves from firing , lack of training and more . The batteries claimed to have shot down five or six German aircraft . However , after reviewing the Luftwaffe ' s own reports and the number of wrecked aircraft in the Oslo area on 9 April , researchers have concluded that probably only one German aircraft was shot down by ground - based air defence . A number of aircraft were hit , some very seriously , but only one seems to have crashed as a result . The guns were abandoned when Oslo surrendered due to the threat of German bombing of civilians . The fate of the M / 32s after the Germans captured Norway is unknown , but they probably used them for air defence in Norway . Today one is preserved at the Norwegian Armed Forces Museum in Oslo .