For decades, I've used commercial floor stripping ammonia, Butches, Sweets, Copper Cutter, and I'm sure a couple of others, of which names I've long forgotten.
All work to a degree, and all that contain ammonia can damage blue & wood finishes.
A few years ago someone suggested Outer's Foaming Bore Cleaner. The original stuff was made in Finland, then later made under license In the USA. The stuff comes in a little wussy-sized spray can with a snorkel tube. It doesn't have any smell, no taste & no hazardous properties, BUT it DOES an amazing job at removing copper fouling. I have numerous Milsurp Rifles in conditions ranging from neglected battlefield pick-ups, to factory refurbs, to virtually unfired.
Whether "new" or fouled by my shooting, all clean up with nearly no effort by simply filling the bore with the foam, then walking away for about 30 minutes. When a patch is passed down the bore to push out the spent foam, it emerges dark blue, indicating that the copper has been dissolved. I only need to repeat treat badly pitted and worn barrels, otherwise, the foam completely cleans down to bare metal & it won't harm bluing.
And that anemic little 3 oz spray can? It will clean over 30 rifles.
In rare instances where a badly pitted or neglected barrel needs repeat cleaning, a nylon bore brush does a great job. Avoid copper or bronze brushes, since the Foam cleaner will attack the copper in the brush bristles.
I've used all the major copper cleaners, & the Foam is by far the best.