The pencil shooter and its big brother, the chest plate crossbow, both need a string to propel the projectile.
I used paracord until now. But truth be told I wasn't happy with the results. The inevitable chafing killed the string far too early, even though I polished all surfaces the string comes into contact with.
So I went to the arbalist guild forum and found the solution: The trusted traditional method.
Watch a master at work here
I made myself a miniature crossbow string rig. This is needed as the procedure requires that the string is under strain why you wrap it. Then I took my 40 kg polypropylene string I always use for pouch attachments, and made this first version of a mini crossbow string from six strands, "served" with a seventh.
It is amazing how it feels, very very hard, and you can even bend it into shape like it would have a copper wire core.
Well waxed, it should survive a long time. I have ordered genuine Dacron, which is a lot thinner, and should give me even better results.