Choreboy wrapped around an old bore brush.
Choreboy on a worn bore brush works well. 50 years ago in the Army I spent several weeks on casual assignment in the post armory at Fort Benning, Georgia. We serviced small arms and crew-served weapons from just about everywhere (western and commie-bloc) used for training purposes, which means a lot of firing by a lot of people.
For heavily fouled bores and chambers we used 0000-grade (the finest grade) steel wool wrapped around old bore brushes, worked dry (no solvent) to cut through heavy metal fouling and powder fouling, followed by normal cleaning with solvents, brushes, and patches.
Since then I have used the same procedure on my firearms, including a 1976-vintage Model 19 revolver that served me for several seasons of competition shooting (thousands of rounds of cast lead bullets every year). Chambers, forcing cone, and bore are still as new condition. Absolutely no damage whatsoever. All the steel wool does is cut through the deposits to remove them, and the steel wool is far softer than the steels used in firearms manufacture, so it cannot scratch or mar the firearm at all.
During my years shooting in the Service Rifle category I have fired thousands of rounds of surplus jacketed ammo through my M1 rifles, Springfield rifles, and a SA M1A National Match rifle. Metal fouling from the jacket materials (typically copper, brass, nickel) can be a serious challenge. Again, a worn bore brush tightly wrapped with 0000-steel wool will make short work of the cleaning job.
I have purchased antique firearms with bores that looked like corroded sewer pipes, and cleaned them up quickly and easily with the same treatment.
Worst case of leading I ever saw in a revolver was a .357 S&W Model 27. Wouldn't chamber a round of ammo. Cylinder would not rotate because of leading on the cylinder face and barrel breach face. You could barely tell that the bore had any rifling at all. About 20 minutes with a bore brush and 0000-steel wool, followed by normal cleaning, the piece operated like brand new, and everything presented as functionally perfect.
I have a small roll of 0000-steel wool in my gun cleaning kit. I think I purchased it about 40 years ago, maybe spent $1.99 or so, and I'm sure it will last the rest of my life.
YMMV