[quote=@Kronshi]
-twine has no elasticity
-you have no leather unless you rip your bag apart, which will be hard because of it's good quality
-you have no skill in making stuff so your sling would suck even if you did manage to make it
[/quote]

Elasticity: You're thinking of a slingshot, the sling David used was braided twine.
Leather: The leather portion of a sling is like 2x2 inches, I have a dagger and I can cut that much out of it without destroying the bag.
I can't make it: It's a matter of poking two holes in the leather square and pushing the twine through.