The problem with using scrounged lead for casting round balls for muzzleloading guns is the unknown alloy. And consequently, its hardness. For a muzzleloading rifle especially, dead soft pure lead is all I will use. I use metal of know purity, and even so, tested it with my Lee hardness tester. It doesn't even register on Lee's chart, being a BHN of less than 8. So to answer your question as best I can, use lead that doesn't register on Lee's chart, and it will be soft enough for your purpose.
Incidentally, our wheel weights (North Central BC) is BHN 11-12, and they all are lead alloy, even the ones with zinc included. NEVER USE WW WITH A 'Z' MARKED. It will destroy your mix and the pot in which you have melted it. You will never be free from zinc from that day onward.
I was fortunate, a couple years ago, to have a friend whose task it was to remove all of the lead sheeting from an entire x-ray lab...a whole floor! So he gave me around 700 pounds of the most beautifully soft lead there is which I cast, over a period of a couple days, into 1 pound ingots.