Abnormalist wants a Sturmgewehr 44. The first modern assault rifle, developed by Germany during WWII, and luckily not made soon enough or in large enough numbers to affect the outcome on the battlefield. It is chambered in 7.92x33, a shortened version of the 7.92x57 (8mm Mauser), making it developmentally the forerunner of all the intermediate cartridge weapons that followed.
There are a few of them in private hands with paperwork to make them transferable. The one I found was going to sell for over $100,000.