... back in those days there wasn't any copper solvent you could leave in the bore without risking pitting. ...
MD-
"Copper solvent" covers a multitude of commercial concoctions. Some could certainly pit a bore if left in it for a while, back in those days. However, Hoppe's No.9 could be left in a bore almost indefinitely. I'm not sure whether it meets your definition of "copper solvent".
The copper solvent in No.9 used to be ammonium stearate, but nowadays it's ammonium oleate. (Both are derived from long-chain 18-carbon fats, with the stearate completely saturated and the oleate mono-unsaturated.) These aren't the fastest acting solvents, but they will eventually get the job done.
Old timers used to clean their rifles after hunting season, and leave some Hoppe's in the bore during storage, mostly as a rust preventative. After some months just before the start of the next season, when they ran a patch through the barrel they would be amazed at the gobs of green goo that came out of the bore on the cleaning patch.
Elmer Keith recommended rifle storage with Hoppe's in the bore.
No.9 Benchrest uses the same solvent, but it's augmented with one of the synthetic amines that also removes copper. It cleans somewhat faster than No.9 in my experience.
--Bob
