Mine don't copper too much, but I clean them after every use. I use shooter's choice, but I think they re-named it. I'll have to see what my new bottle says. MC-7 "Shooters Choice". Been using it for years with good results.
Shooters Choice has been left behind. Try some Montana X-Treme. It works quite well, and it's safe to let soak in the bore.
I don't like the name, everything extreme this and extreme that these days, but the stuff works.
