lol if you're starting with one of joerg's creations you can't go far wrong :-)

Now if someone would just create a printable verison of his semiauto pencil crossbow. I'd be a very happy aardvark.

So what's a whisker biscuit and why can't you make your own ?
I think it's the thing that holds the arrow in the centre of the circle.
Don't see why you can't make one with a material like polyflex or semi-flex.
Looks like a pretty simple design.