The launch barrel is made from the cap of a large marker. Ensure that the cap you use is a larger diameter than the ballpoint pen. Create an opening in the closed end of the marker cap by either using a wide bore drill bit or simply cutting the end off the cap. Use caution as this part can be tricky and fingers can get hurt if you're not careful.
Next, snip your rubber band and tape each end to the outside of the marker cap on opposite sides. Use plenty of tape to secure the rubber band in place. Lastly, wind a small amount of masking tape in the centre of the rubber band after it's been attached to the marker cap, this will act as the catch when you arm the grappling hook gun.