Disclaimer: This is a non-fictional weapon that was actually used by Nazi Germany during the reign of the Third Reich. All following traits have been respectfully acknowledged.
Creation in WWII
World War Two was coming to an end: The Allies had invaded most of Europe, and were slowly making their way towards Berlin. In the middle of the war, German and Waffen SS soldiers decided to create a gun, which would soon take its place in history. The StG-44/MP43/MP44 (Sturmgewehr Model 1944), or "Storm Rifle Model 1944" was a heavy calibre weapon. Because high caliber munitions were being further introduced by the Luftwaffe near the end of the war on planes such as the Horton Ho-229, a Wunderwaffe (Vun-der-Vaf-fen, or Wonder Weapon), the Waffen SS decided to create a gun that was lightweight, reliable, and stable in firefights. Thus, the StG-44 was born! However, Adolf Hitler did not like the idea of a high calibre assault rifle, and banned the idea from Germany altogether. But, the SS continued manufacturing, secretly using it in German military operations. To hide it's identity, they gave it an alias, the MP-44 (Maschinenpistole, or Machine Pistol). Maschinenpistole's tended to have a smaller calibre round, such as the MP-40. Hitler therefore assumed it was a small calibre weapon.The weapon later turned out to be an influence on later assault rifles like the M16 and AK47.
Use in the Military today
Accepted in the family of Assault Rifles, the StG-44 inspired the most famous weapon classes today, giving ideas for creating the AK-47 and the M16 series. After the war, it was still used in Western Europe. Because the ammunition was so low for such a high calibre, it is now used with a lower-calibre round. Argentina attempted at making their own version of the StG-44, but later decided to accept the FN FAL as their main assault rifle. It stayed with the Volkspoleizei until around 1962, when it was replaced by the PPSh-41. It was also used by paratrooper divisions in the Yugoslavian military up until the late 80s. Now, newer, more modern models of the StG are being manufactured in Germany right now, and will now use the same calibre munitions it did in its "A day" back in the 1940s.