Originally Posted by crazyWayne
Kerosene and/or later fluid works well
I've found Kerosene to be the best. The main reason is Cosmoline is a petroleum based product. So based on that it will remove better with a petroleum based solvent. Back in the mid 90's I bought a new, unissued Enfield complete with bayonet, still mummy wrapped in the Cosmoline. I removed the bolt and magazine and placed them in a 2 pound coffee can filled with warm, clean Kerosene. It was Summer at the time and I was living in Lake Havasu City, Arizona and the outside temperature in the Sun was close to 120 F.
While the bolt and magazine were soaking I pushed a cleaning rod through the barrel and Cosomoline came out like toothpaste squeezed from a tube. I then used conventional patches and bronze brushes with the warm Kerosene, washing down the entire gun. Afterward I blasted everything with compressed air, and the gun was spotless. I gave it a good oiling and off to the range I went. The gun performed flawlessly. I did the same with 2 Mausers I bought from Mitchell's a time later. They weren't as bad, but the bolts on both rifles were heavily smeared with Cosmoline. After the warm Kerosene and air blast treatment, they were clean and good to go as well. There was zero need to disassemble them. The warm Kerosene dissolved the Cosmoline and it flowed out of all of the nooks and crannies.
You can use WD-40 in place of Kerosene, but be sure to really blow it out of the internals of the weapon because it does leave a slightly gummy residue that Kerosene doesn't. It sure smells better however.