[BigDaddy]

I also found this:

"Surprisingly, a good slinger hurled a stone as far and accurately as a good archer. Roman military texts recommended archery target practice at about 200 yards. Slingers are known to hurl their projectiles even farther, as much as 440 yards (quarter of a mile)."

Sling stones ranged from golf ball sized pieces of lead to rocks the size of a tennis balls, and where tooled round. Slinging was a sort of game to ancient people, who made their own stones, and practiced for fun.

Also,
"As for accuracy, one ancient writer noted that the best slingers "would wound not merely the heads of their enemies but any part of the face at which they might have aimed." Experiments demonstrate that missiles leave a sling in excess of 60 miles per hour. One Roman writer noted that opponents in leather armor were in far greater danger from sling missiles than arrows. Even if the stone did not penetrate the armor, it was capable of inflicting a fatal internal injury."

I couldn't find information on the effectiveness of armor on slingstones, but I'd just assume slingstone aren't AP.