Cerrosafe is not to be discounted. Standard machinist method and often available in machine shop supply stores. A little pricey and permanent. Clean the barrel thoroughly, either way. You could use another metal, like lead, but it is not an industry standard.
Number 2 is "garden sulfur." If you are rich, you can get lab grade sulfur, either works, but garden sulfur used to be $0.50 for 5 lbs in the larger garden stores or was it 10 lbs. I think it is used to "acidify" soils which some plants like. Melt it. Outdoors if your better half doesn't have a sense of humor, even if she does. No possibility of explosion but if it catches fire, IT STINKS and you will be in the "dog house" for a long time. Clean barrel. Shot of "light" something (WD 40 is o.k). Plug barrel with tight patch ahead of chamber. Pour molten sulfur thru a funnel. (Rolled Aluminum foil works o.k. I think paper might. Not that hot.) Let it harden. Minute or two. GENTLY push it out. It is not as durable as a metal but if it breaks, start over. Any machinist can mike and read off this. Handle gently and you have a permanent record. And in a couple hundred shots, do it again and watch your throat recede. luck.
