It can be used as a weapon if you have a powerful enough sling and a weighty enough projectile (even though it’ll decrease range, but not by much) it also depends on where you hit the person, what they’re wearing, etc. So yes, your statement about them being ineffective is false.
However as Maxey said, I would rather just have an aluminum baseball bat behind the counter.
Edit: You also seem to be confusing a slingshot (the modern version which is from the 1900’s on up) and the Sling While similar devices, one’s been around A LOT longer than the other.