congrats!
bear in mind that it is an offensive weapon used to soften ranks before marauding sword/axeman divisons ... if a stone was whizzing within a metre of my scone every few seconds I think I'd be keeping my head down like a good little pleb.
my targets are either small groupings of trees 4-8m tall and 3 m at the base... or a 44 gal / 200 l drum at around 50 m. Been working closer of late.
I figure if i had to actually hurt someone at that kind of range my reach and tenacity and knowledge of the lay of the land would serve me better than a length of string... but if someone had to pick between two houses, one where fist sized stones kept whizzing out the front door at hundreds of klicks an hour and another where that wasn't happening.. guess which they'd pick

Accuracy matters less when you only have to think about the end of a hallway.
I wonder how long it will be, in my country at least (land of legislation! if you enjoy it, it's probably illegal, or so taxed it may as well be ) before slings have to be stored in a safe bolted to the frame of your house... some tremendous force there. I think I'd take an amateur with a .22 over a pro with a sling as an adversary anyday.
congratulation again on your progress Iron and Donnershlag. you're right to feel proud of yourself... who else knows how to do that these days?