A brown, wax-like petroleum-based corrosion inhibitor often used to preserve firearms for long term storage. Cosmoline is petroleum-based rust and corrosion-inhibiting grease. The consistency of cosmoline varies widely but is mostly found as grease or wax.
The original Cosmoline was an ointment and was used for many different cosmetic and medical purposes. Cosmoline became widely known when it received a government specification as a rust preventive and began its use by the military to protect various equipment from rust and corrosion. Cosmoline could be found on military equipment in the Spanish–American War, World War I, World War II, and the Korean and Vietnam Wars.
firearm preservation
Cosmoline can create a dangerous situation with your firearm. Primarily when found in the bore and fire control area. If planning to shoot a firearm that was stored in Cosmoline , leaving cosmoline in the bore can ruin your firearm or, worse yet, cause injury to the shooter. It should be treated as a bore obstruction and removed prior to firing. Cosmoline found in the chamber area can cause failures to extract.