Then again, I don't think anyone's really proposing an effective range of 300 meters for slings or anything else short of an artillery weapon for that matter. The maximum range a hand weapon could send a projectile to and the effective range - especially against armour - are two very different things after all.

As for the terminal impact thing, I suspect sling bullets are by far more like musket balls (albeit with generally by far better aerodynamic performance) than arrows. Even with their über-shitty flight characteristics heavy musket balls had notably more penetrative power at long ranges than arrows (although hitting with the unstabilized things was another issue), and retained their energy relatively well due to the combination of high mass and compact size. The same basic principle lies behind modern large-calibre sniper rifles AFAIK.

Although contemporaries observed balls hitting from a long range felt "like strong punches" and just caused bruising. Then again, the ability of arrows to cause more than superficial injuries - especially against armour - at long distances wasn't exactly the best either...

But alas the RTW engine does not account for the decrease in projectile effectiveness with distance, so...