I've been shooting some HBN coated bullets in a few rifle barrels also. It is notably reducing copper fouling in the barrels I've been shooting. In barrels that copper fouled very little, I'm not seeing any copper streaks at the muzzle after 20rds down the bore. Even a factory barreled AR-15 that used to copper foul a fair amount, after 16rnds fired it's difficult to see copper at the muzzle. What I do see is evidence of the HBN coating the bore surface. This reduced fouling is nice in my hunting rifles, as I'll be able to leave the bore fouled over a greater number of shots fired, and won't have to clean and re-foul the bore as often prior to heading out on a hunting trip. I don't worry much about leaving a bore fouled when its stored in my climate controlled house.
Your bore's surface is probably pretty rough. I haven't shot HBN bullets down bad copper fouling bores, so I'm not sure what improvement would be gained. I'm confident it would help, but to what extent I have no personal experience with those bores/barrels.
If you have someone examine your bore with a bore scope, you may be inclined to replace the barrel, if copper fouling irritates you.