I've read and been told that the stick on weights are at or closest to pure lead. There are ways to heat to help separate alloys but not 100% effective. Alloys can harm the barrel but considering you use patched ball in rifle the issue is pretty much mute. If you use them use a size that allows the thickest patch. I would not advise using for minis, reals or revolver.
Depending on your area pure lead can be bit of chore to acquire. Some close to coast can find scrap pure used for keels. Sometimes you can find scrapped plumbers lead used in older buildings for shower lining and vents. I've heard of people getting it from old medical or hospitals that are being demolished. The xray rooms are shielded with it.
Some use the stick on even though they might be a bit harder than pure but soft enough they are safe to use. I have searched all recyclers and shops in my area and find robometals my best choice. I check their site till there is a sale and buy just enough to get free shipping. Amounts to 9, 5lb bars and it's 99 pure. My last amount totaled 90lbs over 2 months and at my age I probably won't need any more.