This nerf turret is powered by two motors I found in some old broken rc cars, along with some lego tires, cardboard, and a slide switch.
When the slide switch is on, voltage passes through to the motors, which spin in way so that the dart is launched forward. Toothpicks are used as a stand so that you don't have to hold it yourself, just feed darts through the hole. 12V batteries are recommended because they can launch your dart at the highest speed (even higher are better). Space out the wheels (or whatever other round object) so that the dart can't go through unless pushed hard through. If you want, you can add a gun barrel for increased accuracy, but this may slow down the bullet speed. There are adjustments that you can make to shoot different projectiles as well.