I got a fair amount of copper fouling in my barrel. Ordinary Hoppes No.9 comes out green after a while but hardly touches it. I can see the copper color in my muzzle.

Any recommendations?

Anybody else have this problem, and if so, does it screw up accuracy pretty bad? Shooting ball ammo, maybe 7 of 10 shots go into 2" at 100 yards -- tolerable -- but the other 3 rounds go flying off so my group is like 5 or 6 inches. At 200 yards I can hardly bring myself to call it a "group."

You can try Aqua Ammonia, should be able to get it from a pool supply or printing supply shop. It's about 26% strength so make sure you use it in a well ventilated place and wear safety glasses. Mix a small amount of dish soap in with it to make it stick.

When your done cleaning your barrel make sure that you oil it.

There are other copper solvents out there if you don't want to go this route. I use Boretech copper solvent (about 10%) and have had good results.

Im a firm believer in my new cleaning methods since i was turned on to them.  I use Mpro7 to get rid of all the powder and other soot in the barrel.  To remove the copper i then use Sweets 7.62 copper remover.  After that Gun Scrubber is needed  to neutralize the sweets.  This combo gives you a perfect bore every time