The weapon is a Sturmgewehr 44 (commonly called the MP44). It was made by Nazi Germany in 1943. At the end of the war, both the Soviets and the US claimed large numbers of these guns that were never used (almost like someone was running out of people). The weapons gradually worked there ways through the intelligence apparatus of both countries, and the rest is history.
That thing is way too big to be a MP44, the grand daddy of all assault rifles. Idk what it is but not that, I think