Here a trick, it work good, unwrap the rifle from the brown paper, take clear plastic (Saran wrap) and wrap it tight, just leave a little opening at the muzzle, put muzzle down in a carbadge can , put in direct sunlight on a warm day, wait a couple hours, after 90% of the cosmo will have drain at the muzzle.
Classic: https://geekprepper.org/remove-cosmoline-with-a-solar-oven/
Thanks for reminding me of this trick caramel