Shot is often "hardened" by alloying with Antimony. Premium shot is often as much as 6% Antimony
Arsenic has been used in lead shot as a hardener for centuries.
"Arsenic hardens lead, and is used in alloys for making lead shot and in lead-acid battery plates. It is also said to make lead shot more spherical" http://www.du.edu/~jcalvert/phys/arsenic.htm
"Arsenic is used with lead (0.5% As) in making lead shot. It makes the metal harder than the pure lead and improves the properties of the molten material". http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/as.html
"Inorganic arsenic is a deadly poison and a well known carcinogenic substance. It is used in the making of all lead shot." http://www.medicalnewstoday.com/youropinions.php?opinionid=5352
"*Tip: Lead shot has .5 to 1.0% arsenic (depending on the manufacturer) and can be used as a hardening agent when heat treating lead and lead-antimony alloys. 1/4 of 1% arsenic is all it takes. Adding any more than this adds nothing & will not further harden the alloy." http://www.lasc.us/CastBulletNotes.htm
Decades ago, I routinely cast my own fishing sinkers from scrap lead, indluding recovered shot & bullets. I had to be more careful of asenic fumes boiling off than I ever worried about the lead itself.
I'm NOT worried about this, but rather wonder how The Press will fulinate about, "If he didn't kill the poor man by gunning him down, then he may still kill him by slow poisoning...."