Compressed air first, then if that doesn't work do the powder through the nipple hole trick. Unless there's a tightly patched ball stuck in there, there won't be a tight enough seal to use the grease gun. The grease will just flow around the cleaning patch and make a big mess. 4-F flintlock priming powder would work best, but 3F can be worked into the nipple hole with a toothpick or an ice pick.
