
Originally Posted by
Wikipedia
Ancient authors seemed to believe, incorrectly, that sling-bullets could penetrate armour, and that lead projectiles, heated by their passage through the air, would melt in flight. In the first instance, it seems likely that the authors were indicating that slings could cause injury through armour by a percussive effect rather than by penetration. In the latter case we may imagine that they were impressed by the degree of deformation suffered by lead sling-bullet after hitting a hard target.