On cleaning copper, Sweets is good but you have to follow the instructions exactly. Have used milder stuff like Hoppes, Butches Bore Shine, etc.
Unfortunately, I enjoyed a hobby of restoring old military rifles for awhile and a lot of the shooter grade had absolutely filthy and fouled barrels. I've used everything from a homemade electrolysis kit, JB bore paste in both grades, choreboy copper pads, etc. to clean them up for evaluation. The worst was a poor old WWI era SMLE that had green paint on it, cut stock, but original matching bolt and receiver. I bought it to restore to wall hanger as you could not see any rifling in the barrel. But, I used almost everything to clean it--started with a long soak in odorless mineral spirits, then initial scrubbing with a helical steel bore brush, then more soaking,then powder solvent, copper solvent, jb bore paste, then electrolysis. Finally got the barrel down to bare steel and while it is pitted, this is a shootable rifle again--turns out also it was an early shopped SMLE (or Sht. L.E. or No. 1, Mk III* if you will) (SSA made) which are pretty rare.
The other is a GP 100 that eats lead bullets and leads up quickly. Lewis Lead removal tool is what you want for lead. Ended up only firing plated or fmj through it and used my Smiths for lead.
Summary, most of the stuff works, some stuff is more toxic, harder on barrels, or requires careful techniques. Use the gentlest stuff on new barrels and work up from there.