The Bofors 40 mm gun is an anti-aircraft autocannon designed by the Swedish defense firm of Bofors Defence. It was one of the most popular medium-weight anti-aircraft systems during World War II, used by most of the western Allies as well as by the Axis powers. The cannon remains in service as of 2012, making it one of the longest-serving artillery pieces of all time. It is often referred to simply as the Bofors gun.
The Swedish Navy purchased a number of 2 pounder Pom-Poms from Vickers as anti-aircraft guns in 1922. The Navy approached Bofors about the development of a more capable replacement. Bofors signed a contract in late 1928. Bofors produced a gun that was a smaller version of a 57 mm (6-pounder) semi-automatic gun developed as an anti-torpedo boat weapon in the late 19th century by Finspång. Their first test gun was a re-barreled Nordenfelt version of the Finspong gun, to which was added a semi-automatic loading mechanism.
Testing of this gun in 1929 demonstrated that a problem existed feeding the weapon in order to maintain a reasonable rate of fire. A mechanism that was strong enough to handle the stresses of moving the large round was too heavy to move quickly enough to fire rapidly. One attempt to solve this problem used zinc shell cases that burned up when fired. This proved to leave heavy zinc deposits in the barrel, and had to be abandoned.
In the summer of 1930 they began experimenting with a new test gun that did away with controlled feed and instead flicked the spent casing out the rear where after a second mechanism reloaded the gun by "throwing" a fresh round from the magazine into the open breech. This seemed to be the solution they needed, improving firing rates to an acceptable level, and the work on a prototype commenced soon after.
During this period Krupp purchased a one-third share of Bofors. Krupp engineers started the process of updating the Bofors factories to use modern equipment and metallurgy, but the 40 mm project was kept secret.
Finnish Bofors 40 mm. This gun mounts the original reflector sights, and lacks the armor found on British examples.
The prototype was completed and fired in November 1931, and by the middle of the month it was firing strings of two and three rounds. Changes to the feed mechanism were all that remained, and by the end of the year it was operating at 130 rounds per minute. Continued development was needed to turn it into a weapon suitable for production, which was completed in October 1933. Since acceptance trials had been passed the year before, this became known as the 40 mm akan M/32. Most forces referred to it as the Bofors 40 mm L/60, although the barrel was actually 56.25 calibers in length, not the 60 calibers that the name implies.
40 mm Anti-aircraft Gun
In order to supply both the Army and Navy with much greater numbers of the guns, Chrysler built 60,000 of the guns and 120,000 barrels through the war, at half the original projected cost, and filling the Army's needs by 1943.
Over the lifetime of the production, their engineers introduced numerous additional changes to improve mass production, eventually reducing the overall time needed to build a gun by half; most of these changes were in production methods rather than the design of the gun itself.
There were many difficulties in producing the guns within the United States, beyond their complexity (illustrated by the use of 2,000 subcontractors in 330 cities and 12 Chrysler factories to make and assemble the parts). The drawings were metric, in Swedish and read from the first angle of projection, with lower precision than needed for mass production. Chrysler had to translate to English, fix absolute dimensions, and switch to the third angle of projection.
"It should be noted that the USN considered the original Bofors Model 1936 design to be completely unsuitable for the mass production techniques required for the vast number of guns needed to equip the ships of the US Navy. Firstly, the Swedish guns were designed using metric measurement units, a system all but unknown in the USA at that time. Worse still, the dimensioning on the Swedish drawings often did not match the actual measurements taken of the weapons.
Secondly, the Swedish guns required a great deal of hand work in order to make the finished weapon. For example, Swedish blueprints had many notes on them such as "file to fit at assembly" and "drill to fit at assembly," all of which took much production time in order to implement.
Thirdly, the Swedish mountings were manually worked, while the USN required power-worked mountings in order to attain the fast elevation and training speeds necessary to engage modern aircraft. Fourthly, the Swedish guns were air-cooled, limiting their ability to fire long bursts, a necessity for most naval AA engagements.
Finally, the USN rejected the Swedish ammunition design, as it was not bore safe, the fuze was found to be too sensitive for normal shipboard use and its overall design was determined to be unsuitable for mass production. US manufacturers made radical changes to the Swedish design in order to minimize these problems and as a result the guns and mountings produced in the USA bore little resemblance their Swedish ancestors."