and here I thought the lead issue (paticularly with indoor ranges) was from the lead particles coming from the bullet due to heat/pressure of firing and the nature of a bullet being sheared by the barrel rifling while spinning up.

Ever notice the difference in firing cast lead bullets versus jacketed bullets? Even using the same powder and primer, the amount of blast particulates are much more with cast lead. This is the reason many rounds these days are total metal jacket including the base to eliminate the exposed lead. The amount of lead in the primer is fairly small relative to that contained in a bullet.

There is considerable lead residue in a barrel that has been firing cast lead bullets -- lots. Nitrile gloves are recommended.

That's all my opinion from personal observations. If I am mistaken in my analysis of what I saw, please correct me.