One look at this chart and the weight of tungsten compared with other metals and it explains why TSS (tungsten super shot) is so effective.
Copper is a lot cheaper than heavier metals, has less weight than lead but considerably more weight than steel, and copper isn’t nearly as toxic as lead, so copper may be a fairly good alternative to steel shot?
Looks like irridium would be killer! 
This is interesting napkin math shows cost to produce a case of copper shotshells (250) it roughly $25 more than the cost to produce a case of lead shot, even less of a difference for steel shot. The markup to buy them is significantly more than that.
I am currently shooting boss bismuth’s. I’m not sure if I will shoot boss copper. I may just buy cases of Rio and send it.
