This is the turret that we designed for competition in this years Bunnybot scrimmage. It has two firing mechanisims, one (on the bottom) is a Nerf Swarmfire, the second (on top) is a custom piece designed to fire at close range. The turret itself is interesting as it expands at the start of the match to allow for a better firing angle on the custom gun.
The majority of the custom firing mechanism I machined from some MDF. The clip is machined from nylon. Kyle completed the whole deal by turning aluminum stock to the correct diameter and taping a whole on one end to screw on to a pneumatic cylinder.
The only modifications that we made to the Nerf gun is bypassing all of the circuitry and wiring it into a spike.
Here is the Youtube link 