What is a sling
A sling is a weapon made by two bits of cord and a piece of leather cut to held a stone. It's used mainly by otters, but others use it too. You can read more about it here.
What is a slingshot
A slingshot, on the other hand, is a y-shaped stick with a cord tied at both ends; they're different. I think Brian Jacques, as well as the artiests. they got a little mixed up but you can't blame them. (I only know cos I sling for sport)