The Fedorov was the first practical assault rifle, predating the German Sturmgewehr by thirty years. It was a select-fire rifle with a 25-round magazine of 6.5x50mm cartridges. The caliber was chosen for its light recoil and easy access to purchase from Britain. It was light enough (less than 10 pounds) for easy use and used a cartridge will light enough recoil to be controllable and effective when fired from the shoulder (unlike contemporaries such as the BAR).
You can see all out photos on the Fedorov page in the Vault.