Industrial strength ammonia (janitorial grade at hardware store) works fine. I leave it in a few minutes at a time, works
great, dirt cheap. A couple of Hoppe's #9 patches afterwards and done. CR10 is great, too.
Hoppes #9 will take it out, but slowly.
sweets will get out copper better than anything. however, it can ruin a bbl. faster than anything else. i used it many years ago; not anymore. i don't think that there is a competive benchrest shooter that will use it. i belive that many use Wipe-Out now
I have a swedish Mauser that copper fouls easily. Strong ammonia from ACE or OMC Engine Tuner both work well. I leave them in the barrel of 30 minutes max, pour out the solution and check its colour, and repeat as necessary.
I don't shoot many jacketed bullets, other than 223/5.56 and 22-250 in my rat rifles. Fortunately, none of these barrels copper-foul very badly. I got away from using Sweet's or other 30% Stronger Ammonia-based cleaners, out of concern that dozens of 15-minute dwell durations of these liquids on metals might equate to an overnight stay and resulting metal etching or damage. I use the "Wipe Out" bore foam after a hunt, then clean with Hoppe's #9 and leave the bore damp with Hoppe's and the rifle muzzle-down in the safe. I check the bores every month or so, patch out the Hoppe's and it is always at least slightly blue. I don't think any reasonable method ever gets ALL of the copper fouling out of a barrel's metal--I think it is a condition that you manage reasonably to maintain an accuracy standard, and call it "Good" from there.