Is that a 20mm or 30mm cannon attached to that DB?
Looks like people have provided you with some of the best visual aids. As you can see the crankshaft rotates the propeller shaft which is hollow and runs through the entire engine. On the rear-end of the engine is mounted the cannon that fires through the hollow propeller shaft that runs through the entire block. From there it's on to the propeller and the hub. The cannon's barrel sits stationary inside the hollow rotating propeller shaft and the entire cannon assembly is mounted to the engine block.