Egads, I havent paintballed since around 1987, not long after
one of the first entrepeneurs in our region leased a large wooded field an hour east of me. Players at the time rented and shot a bolt-action cattle-marking pistol called the "Nelspot 007", which has long since become obsoleted by semiautomatic versions with larger CO2 tanks with large hoppers. For typical play attire (t-shirt and outer cammo shirt), the impact is comparable to getting hit by a bumblebee while riding a motorcycle at 60 mph ... it'll leave a smallpainful welt, and a hickey-like bruise, but wont break the skin. Anyway, I dont want to bore people with nostalgia.
Getting back to your question: I wouldn't trust them in a slingshot, dude. Although fairly strong, those balls are designed to break on a sturdy impact. Picture bath pearls, filled with water soluable dye instead of lavender-scented glycerine. You might be able to shoot a few, if you dont squeeze or pull too hard, but if one breaks in your sling, it'll wreck the pouch. It's not worth it.