According to Machinist's Workshop, April 2007 issue, Kano Kroil was the best commercial penetrating oil for breaking free rusted nuts. Liquid Wrench took only about 20% more pressure, but costs only about 1/5 as much. However, the best penetrate they tested was one that I have used for years: a 50/50 mix of ATF and acetone. This homebrew mix took only half the force to free the test pin. The article even suggested power steering fluid would work. (It is actually usually the same stuff, only pricier in the bottle labelled for power steering.)
I keep some pre-mixed in a relabelled acetone can (about the same size as the Kroil one). For ATF, I use the Ford type (there is some chemistry behind this, not that I have any love for Ford). It does work well to soak a barrel -- I plug one end (usu. the breech) with a silicone stopper, carefully pour the mix in with a funnel, and plug the other end with another stopper. You have to seal both ends, otherwise the acetone will evaporate. Yes, this does seem to release lead, copper, and a lot of other goop from the bore. All with little effort and abrasion.
This mix also works really well to free those stuck choke tubes. Soak for a week, then apply muscle. Only once did I have to apply heat to get a stuck tube out after using this solution.
If I want to use it with patches and strip out copper, I mix a batch with about 10% commercial ammonia and some Xanthan gum to thicken it. After getting the bore as clean as I can, I start running this mix through with a mop until the whole thing is coated. After getting a couple patches without blue or green on them, I repeat the mopping. If I seem to have gotten all the copper out, I then take the barrel to the tub and wash it inside and out with water and Simple Green. After drying it with a hair dryer, I suggest running some CLP or equiv in it, on it, over it, around it, etc.
BTW, I use brake fluid for soaking parts having a little surface rust for a few months before I buff them. Brake fluid can actually reverse very light rust. It also stops rust from continuing. (Not much oxygen or water submerged in there...) You can take a little rust off of a barrel with brake fluid and light rubbing with 0000 steel wool. Sometimes I'll throw some brake fluid on something until I can get back to it -- even if just to keep it from rusting. I have a few 1 1/2" and 2" pipes, capped on one end and have a screw-on cap on the other, which I keep filled with various fluids. One has brake fluid and is often a parking place for a barrel after cleaning (and drying with aforementioned hair dryer) until I get the action rebuilt and ready for dealing with the metal finishes.
Oil Treatment (like STP) mixed with a little acetone can also be used to make a decent CLP-like fluid. We used to use that mix in the action of a firearm. The Oil Treatment is sticky enough to cling to the metal and thinning it with acetone makes it easier to get a thin coating. (It also cleans the surface a little, helping the adhesion.) I would put straight Oil Treatment in the bore when putting a gun away for a while. Mobil 1 gear oil (the heavy weight POA stuff) also makes a good lubricant. I have pretty much given up on these STP/gear-oil solutions since there are good products on the market that don't break the bank (CLP, Rig, etc.), but I use the ATF/acetone mix almost every week.
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