Having done this many, many times over the years, I have hit on the easiest, cheapest, and least messy way of doing the de-Cosmolining.
Use ODORLESS MINERAL SPIRITS,(it is very cheap, and easy to get at the hardware store, paint store, whatever)
A stiff paint brush or parts brush, a long trough (like a plastic hot water tank used in bluing works best) but a bucket will do if you are going to do it outdoors.
I have even lined a trench dug in the ground with black plastic sheeting, filled it with Mineral Spirits and used that. Worked great.
You could also make a long, shallow box of wood, and line that with plastic sheeting.
Soak all of it that you can easily disassemble overnight, then go after it with the brush. You can use a plastic spatula to scrape off the really heavy deposits before you begin soaking/scrubbing.
Do not soak the stock.
I often use a plastic Tupperware type collander to hold small parts while soaking and brushing.
Discard the gooey mineral spirits/Cosmoline slop, and go over it again with clean stuff. Do the bore with an old brush and the Mineral Spirits. Use a toothbrush on little nooks and crannies. The type with opaque handles work better than the ones with clear handles, as the opaque handles are more chemical resistant polypropylene or nylon as opposed to the styrene used in the clear ones.
Mineral spirits won't hurt the stock, and can be used to wipe it down as well.
After this all dries, there may be a bit of white residue on the gun, but regular cleaning with Hoppe's or Shooters Choice will easily remove that and finish the cleaning process properly.
After it is all clean, be sure to oil it down well with a good quality corrosion inhibitor. I use the Shooters Choice FP-10 for almost everything, but Break Free works well too.
I also recommend wearing eye protection, as the splatters of chemical can be very irritating if they get in the eyes. Use good ventilation, and keep the chemicals away from heat and flame sources like hot water heaters, etc.
The used chemicals can be disposed of wherever you take your car oil for recycling.
Just mix them with the used oil before you take it in. Safe and enviro friendly.
you can aslo recycle it by using the Cosmoline/Spirits to coat other outdoor stuff that you want to protect against rust.
If you want straight cosmoline, let the Spirits evaporate after straining it all through a T-shirt lined funnel to remove grit and grime from the fluid.