I use Liquid Wrench to clean powder fouling and sweets 7.62 for copper fouling.
I'll apply Liquid Wrench liberally on a bronze brush, then run it through 10 times adding more oil every 4 strokes. Run a dry patch through.
Then I'll apply the copper solvent liberally on a nylon brush, run it through 5 times applying solvent to the brush each time. Run a dry patch after it sets 5 minutes.
I'll repeat the penetrating oil procedure until the patch is clean. It usually only takes 3 or so episodes of penetrating oil total.
Then I'll apply motor oil as a rust preventative.
I make my own patches from t-shirts.