I read several different methods that work on rifles but they all sound like they would be bad for leather. The most popular is boiling water or a steam sprayer. It doesn't take a lot of heat to melt cosmoline. Also popular are a blow dryer, mineral spirits, brake cleaner, and other solvents. Hot water might not be too bad but solvents destroy leather. If you google cosmoline removal you may be able to find a good way for leather. There are lots of articles but of course they all seem to be about the guns themselves.