The MG 15 machine gun served the German Luftwaffe (Air Force) heading into, and during, World War 2 (1939-1945). It was derived from a 1932 initiative and born from the existing, earlier MG 30 model that originated in Switzerland. The design was standardized as an aircraft machine gun through the MG 15 (flexible) and MG 17 (fixed) products and also influenced the famous land-based MG 34 and MG 42 General Purpose Machine Guns (GPMG) of the German Army that followed. As an aircraft weapon, the machine gun was designed for mounting atop a system to be used by aerial gunners for engaging targets around the aircraft. These weapon stations were set throughout a given bomber type aircraft that required multiple crew positions (as in the Heinkel He 111). The weapon was air-cooled, fully-automatic and operated from a recoil action while chambered for the German 7.92x57mm Mauser rifle cartridge. Rate-of-fire was up to 1,050 rounds per minute with a muzzle velocity of 2,480 feet per second. Sighting was through an iron arrangement while feeding was through a 75-round double-drum, cassette-type system. The weapon weighed 27lbs when loaded and featured an overall length of 42 inches with a 27 inch barrel.
NOTE: The MG 15 model machine gun shares a similar designation to the MG 15nA of World War 1 (1914-1918) fame though they remain two different weapons at their core. The 1930s revision was headed by Rheinmetall while the original gun fielded on World War 1 aircraft was by Bergmann.
Development of the MG 15 occurred during the interwar period following World War 1 and preceding World War 2. During this time, the German war machine was kept in check due to the limitations placed by the Treaty of Versailles signed after World War 1. However, many workarounds emerged and one was Rheinmetall's stake in neighboring Solothurn Waffenfabrik AG of Switzerland beginning in 1929 -this allowed the company to continue production of weapons deemed illegal by the Treaty. This allowed the MG 30 to come into being and set the foundation for the MG 15, MG 17, MG 34 and MG 42 lines.
In 1932, Rheinmetall continued work on the MG 30 and produced a lighter-weight version which became the MG 15. This design featured an open-bolt action for improved automatic fire with feeding from the 75-round dual-drum ammunition supply. The dual drums fed the weapon in an alternate fashion - one cartridge from one drum and then the other - which promoted a better-balanced system as each ammunition cassette was emptied in turn and lightened the weight of the weapon evenly. The weapon was designed to be affixed to a flexible/trainable mount and sighted through an iron arrangement. A fixed installation along these same lines then emerged as the MG 17.
The MG 15 was of a generally simple appearance, utilizing a tubular receiver housing the action. The trigger unit was slung under the receiver in the usual way with a large, oblong trigger ring suitable for gloved hands (two trigger-finger grooves). The pistol grip was solid and ergonomically-shaped though there was no shoulder stock for additional support. Ahead of the trigger group was the feed mechanism to which the drum magazine was a saddled over along both sides of the receiver. The receiver then tapered some to shroud the barrel in a heavily perforated heat jacket. Sights were affixed at the jacket. Aboard German bombers, ammunition drums were stored within reach of the machine gun station and could be quickly changed out as the combat situation warranted. In practice, the guns proved their worth and their long design allowed for much of the recoil action to be absorbed, making them suitable full-automatic weapons against incoming enemy aircraft.
Despite its roots as an aircraft gun, the weapon was utilized in a ground support role when the dire German situation required it - particularly following the shortages of 1944. The MG 34 was already in widespread use at the time but available stocks of MG 15s meant that, they too, could be slightly modified for a similar land-based support role. Modifications added a single-strut shoulder stock at the end of the receiver, a bipod assembly just aft of the barrel, revised iron sights for ground-based firing and a simple carrying handle ahead of the drum magazine. Overall, the major design elements of the airborne machine gun remained which, of course, made it an overly long, cumbersome and heavy weapon in practice do infantry. It saw limited issue and was not a well-liked ground support machine gun for Axis troops. It's one strong quality was its "straight-line" configuration which aligned the shoulder stock with the action and barrel, inherently lowering "muzzle climb" which made them accurate at range. The action remained faithful to the original airborne version which proved reliable in the heat of battle.
The MG 15 series was eventually superseded by larger-caliber systems coming online around 1941 but the German war-time need still pressed the weapon into service due to available stocks. The weapon was adopted by the Japanese military and given the designation of "Type 98".