Internal mag, you didn't have to wait until you were out to reload. A little more info. Only about 30,000 of these were made and only 10,000 were issued to Devil Dogs. They were unique because the barrel could be removed so the paratroopers adopted them
The US Army tested the Johnson Rifle exhaustively in 1938 at Fort Benning, comparing it's functions with early "Gas trap" M1 rifles. These tests found a few weaknesses to the overall Johnson design but the army was impressed by the concept, inviting Mel. Johnson to re-submit the design for further testing at a later date (see elsewhere on this site). With the failure of the design to win acceptance with the US Army and later the Marine Corps., Johnson successfully sold the rifle, now the M1941, to the Dutch for use in the Far East colonies with their Koninklijk Nederlands Indisch leger (KNIL - The Netherlands East Indies Defence Force) troops who were commanded from the Island of Java, in what is now Indonisia. The rifles and machine guns that were delivered saw extensive use against the Japanese before the Dutch colonies were overrun. The undelivered rifles and machine guns intended for KNIL use were now impounded at Cranston.