Old military rifles, to be precise. I wrote on this subject a while back(here), and have something to add.
When you've got a rifle with the barrel/action heavily greased, especially if the grease is old and getting hard/hardened, spraying with carb cleaner or something doesn't really do it. I recently got a piece of 4" PVC pipe, put a permanent plug on one end, and poured a gallon of kerosene in it. You can take the barreled action out of the stock and lower it into this, slosh it up and down a few times, put a cap on the open end and leave it a while. When you remove the cap, slosh it a bit more and pull it out. Kerosene does a nice job of dissolving old grease and won't harm the metal or finish. Let it sit a while for the volitiles to evaporate and you can either hit it with the spray to clean the remaining oil off, or simply strip it down and clean/oil the parts. If you have to leave it a while before doing anything, the oil left on the steel by the kerosene will protect it from rust. For a while; it's not a long-term protectant.
Additional: if you don't want to have to clean off the kerosene oil, after the piece dries a bit you can spray it with carb or brake cleaner; it'll strip the oil off, and then dry fast. Be sure to use good ventilation and gloves if you do.
Better would be a steel pipe, but I didn't have anything that size and don't have a welder to put a plug on the end. And you should either tie it to something to hold it vertical, or build a rack to hold it. But it does a nice job of getting the worst of the stuff off.