Quote Originally Posted by Gnoman View Post
Don't extrapolate too much from this. The Spanish abandoned a lot of their armor because the environment they were in was so hot and humid. This is well documented. There's also a lot of second-order reports from people who weren't there making assumptions on what happened that have been heavily conflated with the eyewitness reports. Not to mention, of course, that the vast bulk of the forces used by Spain against the Aztecs were locals - the relatively small number of Spaniards allied with many of the Aztec's long-term enemies, who never had late-era steel armor in the first place.

That the Spanish took casualties from slings is almost certainly true. That doesn't mean that the slings punched through armor meant to stop musket and pistol fire. There's a good chance that the casualties in question were unarmored entirely.

If slings could punch through a 16th century breastplate at all, no difficulty in learning the weapon would have stopped ancient empires from imposing sling training by force. Because if a Spanish breastplate won't stop it, absolutely nothing will. Guns probably would never have been adopted at all.
I was also inclined to be skeptical of slings penetrating plate armor, but I do wonder if they could impact hard enough to injure someone in plate armor via concussive force, particularly if they struck a helmeted head. Thoughts?