Items needed: 1 dispenser, 2 blocks, 12 redstone, 4 redstone torches and a lever.

Step 1: Place the dispenser.

Step 2: Place 5 pieces of redstone behind the dispenser.

Step 3: Place 2 blocks beside the fourth piece of redstone.

Step 4: Place 2 pieces of redstone on top of the blocks.

Step 5: Place the 4 redstone torches on the sides of the blocks.

Step 6: On the side of the dispenser place 5 pieces of redstone then a lever.

Step 7: Flick the lever on to turn the machine gun off and off to turn it on.

Step 8: It's easier if you watch the video :P