The captured German MP-40 is fired far, far more times (by the British soldier, the German infiltrator and the Frenchman) than the capacity of its single 30 round magazine.
The credits list Gavin Macleod's character as British Cpl. Tommy Behan. a corporal would only have 2 chevrons on his arm. He is clearly wearing 3 which would indicate he is a sergeant (Sgt).