This is how to make it:

Step 1: Make a fast redstone clock (see photo with repeaters)

Step 2: Place a dispenser 4 blocks away from the clock

Step 3: Place a redstone behindthe dispenser and place an iron block behind the redstone

Step 4: Place a repeater facing the iron block and connect it to the clock

Step 5: Place a piston on 1 side of the iron block and connect it to a side of the dispenser

Step 6: Add a lever where is the redstone connected to the dispenser

Step 7: Fill the dispenser with arrows and READY TO FIRE

Step 8: Test it if its not working see the picture #3

Step 9: Dont forget to add a diamond or add a comment!