Get a bottle of Sweets 7.62 if you can't get the copper out. It's best to get the powder fouling first so the sweets can work. It will absolutely, 100% work. It stinks and it's obnoxious. Be careful to get it all out when you're done. And immediately pass a patch with oil on it through the barrel a couple of times. If you can't get all the copper out in 15 minutes, your barrel is made of copper.
I use a bore guide, turn the gun barrel down, and trickle a stream of sweets 7.62 down the rod and let it get on the brush. Take about 10 strokes back and forth with the brush, give it 5 minutes, do it again, and then patch it out. You'll see that gooey green foam come out.
I bought a Savage 223 varmint rifle that had a pretty rough barrel. It was new. The machining was rough. And it stripped copper like crazy off the bullets. The muzzle would be copper colored after 10 shots. So I shoot 10, strip it with sweets, oil it, shoot 10 more.
It took almost 170 shots before it quit depositing copper. I hated that but that stupid gun will shoot lights out a very long way off.
I can tell you, if you're looking for the nuclear option this is it. Just finish the job in 15 minutes, and immediately oil the barrel. I've never had it hurt a barrel. But I've been careful.
I would recommend a nylon brush. Because the bronze brushes.. guess what! They're made of copper. If you use a bronze brush you'll always get green but it's just eating the brush.
I keep mine hidden in the bottom of the refrigerator. It lasts a lot longer that way. If it gets hot it'll outgas and get weak
