I have a puncher breaker in .22 cal and I tried the rabbit magnum pellets which are kind of like a slug. They did not do well, but that's just one of several options.
Get a hold of Dale that makes Varmint Knocker slugs. Get a sample pack of one hundred slugs, maybe 4-5 different weights and try them out. For $15-$20 it will be worth it to see which ones shoot out of your particular gun. Good luck. Stoti