Known to be the first commercially-successful Submachine Gun in history (Besides the rare Beretta Model 1918), the MaschinenPistole 18, I, was used by the German Empire's Sturmtruppen, in the final year of the First World War, 1918. From there, it would continue to be used in that area after the defeat and collapse of the German Empire, and was even reiterated as the MP 28, II ten years later.
Instead of using its own original magazine, it originally fed by the Luger P08's magazines, Especially the 32-round TrommelMagazin 08, otherwise known as the Snail Drum, although it had poor reliability.
Yes, that means you can put an actual 8-round Luger Magazine into it, but what kind of madman would do that??
Here's some images:
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Note: due to a texture bug, this bullet appears black, I have no idea why
Note: Same issue here, along with the Magazine Well
Download: https://www.mediafire.com/file/tkbgy93dxm4tb61/MP18.zip/file