That's not what David used. An ancient Hebrew sling bears no resemblance to the modern toy. It was a long strap that was swung around multiple times and released, and could propel stones at 150 mph
Not after he became king. Banged a soldier's wife, put the soldier in particular on leave so he could go home and bang his wife to cover it up, he wouldn't bang his wife, so David put him on the front lines hoping he'd die.