Found a good article on non-toxic shot.
"steel [7.86 grams per cubic centimeter] is the lightest of the bunch, followed by bismuth, at 9.60 gms/cc; Tungsten-Iron, at 10.30; Tungsten Matrix, at 10.60; lead, at 11.10; and, the real heavyweight in the ring, Hevi-Shot, at 12 gms/cc."
This really shows that it is not too much about the atomic weight!
"Tungsten-Matrix shot as loaded by Kent is slightly heavier than bismuth and close to the same density as lead. Almost as soft as lead, it too can be used in vintage guns."
"Hevi-Shot [is] composed of tungsten, nickel and iron [and] is the heaviest of the nontoxics. At a density of 12.0 gms/cc, it is heavier even than lead. Owing to its extreme hardness, Hevi-Shot should be used only in guns designed to handle steel shot."
now this has me concerned, as I have been buying it. Currently I have a question in "answers" addressing this concern.