The rifle still has the cosmoline all over, what is the best way to clean this off?
The cosmoline is not only all over it, but all in it as well. The rifle was dipped into it as a liquid, so the cosmoline not only soaks several millimeters into the wood, but in every nook and cranny, all through the bolt, sights, up under and behind every piece. Cosmoline removal is an art and science with military surplus collectors, and everyone has their favorite concoction or method. It really depends on how far you're willing to go and what you intend to do with the rifle. You can start with a simple wipe-down of the rifle's external with mineral spirits and rags. This will clean it up nicely, but won't get it out of all the little places it's gotten. Cosmoline melts at a pretty low temperature, around 140 degrees, so people have constructed "ovens" out of garbage cans and incandescent bulbs to melt it out; some just place it in their kitchen ovens with the temperature dialed down as low as possible; some put them in black plastic bags and let the sun do the work. You'll hear of people using hot water or the dishwasher, but that's generally discouraged due to possible damage to the rifle or dishwasher. Some will go go with pretty aggressive chemicals such as degreasers and hydroxide-based oven cleaning products, again balancing speed and ease against potential damage.
My way is to wipe the rifle down with mineral spirits, dis-assemble it entirely, place the pieces and parts in a tub of mineral spirits, and just clean them part by part with rags and an old toothbrush. The stock is different - depending on whether I'm going to re-finish or restore it, I'll just wipe down with mineral spirits until most of the cosmoline is out, or I'll strip and sand until the wood is where I want it to be, taking care not to round any edges or remove any stamps or cartouches.