It's very good lead to mix with other lead, but way too hard to use for blackpowder guns. The arsenic is in there to make it harden easier (it's kind of an antimony multiplier). Also, lead shot does not have any tin (tin is added for toughness and castability)
See if you can find some scrap roof flashing lead, or shower pan lead. Those should be almost pure lead with just a little antimony. Also if your dentist still uses foil-backed X-ray film, the foil is soft lead.
"The only way to stop a bad guy with a gun is with a good guy with a gun"