Arcturus, adam and deadkelly both gave good info on cleanign that cosmoline.
another method is (make sure your wfie is _NOT_ at home when you do this) to strip the rifle, and run it through on the top tray of the dishwasher.
works great, but that can offend the sensibilities of the fairer sex
I myself went ahead and bought a scunni portable steam cleaner a couple of years ago. works great and not expensive, i think they run $40-60 dollars. works on all kinds of stuff around the house too.
bascially it all depend son how you want to do it, and what you feel comfy doing.