A couple years ago when a similar thread regarding CLR came up, I bought and tried some with little success on a SS barrel.
But I also had a gallon of ZEP Wood Deck and Fence Pressure Wash that contained Oxalic Acid that was meant to be reduced with water for use in pressuring washing wood. I had a shot out Savage SS barrel with the original fluted and bead blasted finish so I immersed it in the full strength acid. Lot's of fizzing and after awhile it started to turn a darker color so I removed it and washed it thoroughly.
I don't recommend it for barrel cleaning, at least not on an installed barrel, but it did a really good job of thoroughly cleaning out any carbon and copper. I cut the barrel into several sections and inspected it carefully. I could see no significan etching of the lands and grooves, and other than the darker color, it was about as clean as possible with absolutely no fouling or residue of any kind. A quick dip in a SS brightener would probably have restored its original brilliance.
I did not repeat the test with a diluted batch, as I didn't have another "tomato stake" barrel to use. From previous work experience I know that a mixture of Hydrochloric and Hydrofluoric acids is the standard for cleaning Stainless steel. Though definitely NOT something you'd want around a wooden stock, or 4130 steel and is flat out dangerous to work with, particularly the Hydrofluoric acid component which can be deadly.
For copper cleaning I now use a foaming bore cleaner. Getting the foam in the bore without also getting it everywhere else has been a problem, but I solved it by taking a cartridge brass from the gun being cleaned and drilling out the flash hole to tightly fit the vinyl tube on the can of cleaner. I fit the brass onto the tube, slide it in the chamber and give it a shot of foam. Works perfectly, especially with the gun oriented muzzle down, and I rinse the brass after use to prevent etching.
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