Mmmmm...cosmoline.... (said in Homer Simpson voice)
Take your guns to a local furniture stripper (seen in the Yellow Pages) and work out a deal to have the entire gun assemblies dipped in the "hot tank" that is used to strip wood furniture. I doubt it will hurt metal finish, so, dip that stuff too. If it works for furniture, it will work for guns.
You might have to have the guns dipped twice, usually the second time (if needed), say about a week later, will be after you have detail cleaned and reassembled the guns, as cosmo can still weep out of wood after an initial cleaning. Bear in mind, cosmoline replaces the natural oils in the woods and permeates every pore of metal- hence no rust or rot.
Cosmoline is a pain to remove, but without it, virually all old milsurp guns would have turned into piles of rot and rust long ago.