Upvote:1
My guess (and we can really do little more than guess) is that he means lead slingstones. John Reid of the Trimontium Trust did experiments indicating lead shot from a hand-sling can be loosed at about 100mph by experianced slingers. So not only would they deform on impact, but they'd probably be a little warm after. I don't have the math or the physics to tell you for sure, but certainly when I hit some lead with a hammer repeatedly or with sufficient force it feel warmer in my hand.
If that holds true it doesn't seem a great stretch, if a lead shot is warm after hitting stone or armor, that something moving even faster would be hotter. (from Lucretius' point of view anyway)