honestly.... take the parts to your local car wash, put a few coins in and blast it off.
OR... get several cold adult beverages, make sure the kids are out of ear shot - you will start at some point to scream obscenities - and get out some break free, mineral spirits, or some simple green... then, put a lot of elbow grease into it. Oh, and clear your calendar for the whole day LOL
I have two rifles that are covered in cosmoline. Currently they're in my bedroom closet. There is nothing better than the smell of waking up to cosmoline!