The vaporization thing has always been a myth outside of atomic trace levels that you see evne at room temperature. As someone with eleven issued patents related to thermodynamics of heat transfer, I can tell you the hot gases in a gun, even at thousands of degrees and tens of thousands of psi, don't have high enough thermal conductivity and mass and heat capacity to transfer enough heat to to the bullet to raise its temperature to anything like its melting point in that length of time. And for lead, the amount of heat needed to melt it is 37 times less than the amount needed to turn it to vapor. I've never seen any sign of melting on a bullet base recovered from a backstop, so it follows that no vaporization is occurring.
As I explained before, you do get muzzle blast gas cutting of dust of the bullet basses. Since bare lead immediately forms an oxide layer upon contact with air, the dust's big surface area can oxidize a much bigger percentage of it than happens to a bare bullet. Since the oxide is more acid-soluble than the metal, and for the other reasons I outlined in my last post, this is likely the hazard they are trying to avoid.
As I explained before, you do get muzzle blast gas cutting of dust of the bullet basses. Since bare lead immediately forms an oxide layer upon contact with air, the dust's big surface area can oxidize a much bigger percentage of it than happens to a bare bullet. Since the oxide is more acid-soluble than the metal, and for the other reasons I outlined in my last post, this is likely the hazard they are trying to avoid.