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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 By 1940, it was largely superseded by cheaper submachine guns such as the MP40, and thus production ceased.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
Classified by developers as an "AR-SMG", the MP34 (along with the MAB 38) 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. While a categorically low fire rate and a lower than average maximum damage renders the weapon a five shot kill at minimum range, the weapon is generally worse at close range but retains its five-hit killing power out to 30m, has lower damage drop-off overall and a higher minimum damage. Its lowest rate of fire in its class at 514 RPM also assists in consistency in hitting targets at medium range, although vertical recoil is on the higher end within its class. However, 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. Also unimpressive is the 20 round magazine when stock.
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, the former reducing horizontal recoil to 0.085. 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 Enhanced Grips, while Light Bolt at Rank 3 pushes fire rate up to a more 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.