There's a million stories that the boys tell, but the one I recall went like this:
1) It involves using allot of very hot water or steamjenny to melt the cosmoline and a whole lot more G.I. soap and water followed up by gallons of GI Bore Cleaner as a solvent to clean the hardware.
2) More modern techniques include using kerosene or diesel fuel as a good solvent to dip both metal and wood stocks in.
3) Some new at-home measures involve putting the wood stock in a dishwasher. The receiver, barrel and assorted hardware still require lots of a good soaking in kerosene.
4) Try putting the entire gun in a black trashbag and placing it in your car or truck on a very hot day might melt a good bit of it.
5) The last one I have heard about: Use spray-on oven cleaner after disassembling and let it soak....then spray it down with a garden hose.
Good Luck!
Jim