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The Steyr-Solothurn S1-100 was a Swiss-Austrian submachine gun that produced between 1929 and 1940. Originally conceived at the German Rheinmetall plant, the S1-100 was produced in Switzerland and Austria under the Steyr-Solothurn outlet. It was adopted by the Austrian Army as the MP 34 in 1934, and was sold to various other countries such as Bolivia, Portugal, China, and Japan, in a variety of different calibers.
The S1-100 was expensive to produce, but was noted for its exceptional quality. It came into limited use by the Waffen-SS during World War II, and was designated the MP34(ö) in German service. The gun was never officially adopted by the Wehrmacht. By 1940, it was largely superseded by cheaper submachine guns such as the MP40, and thus production ceased. 
— In-game description
With very low recoil and high controllability being its primary statistical assets, the MP34 is better suited than most in its weapon category for longer distance firefights, and is distinct amongst SMGs for benefiting greatly from stationary, aimed fire. This is in part due to its unique damage model which has slow damage fall off and higher minimum damage of 15 compared to the category standard 13. Its light vertical and horizontal recoil means the MP34 is capable of consistently hitting targets at medium range in fully automatic fire with minimal compensation, although in spite of having the highest unmodified muzzle velocity in class at 495 m/s this is still quite low relative to other medium range weapon classes, potentially complicating engagements at distance. The MP34's compelling controllability is the result of the lowest rate of fire in its class at 514 RPM, and comes with a similarly unimpressive 20 round magazine when stock. These factors give the weapon a quite poor time-to-kill in close quarters fighting, where it can be out-damaged by virtually all other SMGs, as well as some of the faster firing light machine guns and semi-automatic rifles.
Specializations for the MP34 can be applied to further add to its strength at long range, or partially remedy its weaknesses at close range. On the right path, High Velocity Bullets at Rank 1 can improve performance at extended ranges by amplifying muzzle velocity to 560 m/s, which is complemented further by the accuracy boosting Ported Barrel and Barrel Bedding upgrades at Ranks 2 and 4. Extended Magazine brings its capacity to the SMG standard of 32 rounds at Rank 3. The left path offers better responsiveness and minimal possible muzzle climb with Quick Aim and Recoil Buffer, the latter of which combines well with Light Bolt at Rank 3, which pushes fire rate up a respectable 599 RPM. The Rank 4 upgrade of Bayonet is unique amongst SMGs, and could theoretically serve as a last-ditch defensive measure if caught empty.