Back in the day (mid '80's), for cleaning benchrest rifles, we used KLEENZ, GM's upper cylinder cleaner, as it was useful in dissolving & flushing copper & carbon deposits. It was made by Guardsman Chemical & had the same MSDS sheet as the old Shooter's Choice. Precision Shooting even ran an article about it, much to Marksman's Choice (the owner of Shooter's Choice at the time) chagrin. The only thing we added was 2 tablespoons of red ATF (to stop the infernal SCREETCH of the brush down the bore) & 2 tbsp. of Sweet's (to give it some speedy-up in the copper dissoling department, given the between-match time constraints).
While I now use Wipe-out spray foam, Patch Out & their Accelerator, I have used Mercury Quicksilver QuickClean 2 stroke engine cleaner, when at the cottage after running out of Wipe-out. It is manufactured specifically to remove carbon & copper from a 2 stroke engine's combustion chamber (2 stroke engines used to use, maybe still do copper reed valves). The MSDS for it reads quite similar to the old Guardsman Chemical formulation.
I've also experienced the same thing as the OP, typically when trying to clean old Lee Enfields or European Mausers. After going through the same time-after-time-after-time scenario, eventually, I just said to heck with it, dried the bore & took it to the range. After getting back home & spraying all of the bores firstly with brake cleaner (the good stuff for outdoors use), then after it dries in less than the time it takes to bring it back inside, I run a patch with Accelerator, then spray it with Wipe-Out foaming bore cleaner & like others have said, let it soak for 24 hours, dry patch, then repeat. I've noticed, with happy surprise, that once all of the blue or green (think European tombac-jacketed bullets) is gone, the bore is bright & shiny & there's no more of the black "soot" sitting in the grooves. I think it has something to do with getting the barrel warm from firing, then cleaning again.
I've also tried running a few patches soaked in Kroil in a new-to-me barrel that I'm trying to get clean. Just because I bought a gallon of it & need to use it up in this century.
While powder fouling is black & carbon fouling is brown, perhaps as some have suggested, somebody thought that moly coating, both the bore & the bullets, was the answer to all of their problems. And, as more have said, cleaning moly from a moly-fouled barrel can be a bugger!
Or, it may just be that the PO didn't know squat about cleaning a bore, but followed the old wife's tale about using Hoppe's #9 as bore cleaner. It hasn't been able to clean anything other than loose carbon out of a barrel since they took all of the good stuff out of it years and years ago. And since they no longer use sperm whale oil in it, it doesn't even qualify as good cologne.