Bigbull hit the nail on the head. Each individual bore is different and will collect different amts. of copper. I have a Bwng A-bolt in 325 that is terrible for leaving copper residue in the bore. It ususally takes me about an hour to clean it. And I have a Savage 30-06 that doesn't seem to collect any fouling. Usually 4 or 5 patches plus an oiled patch and the bore is clean and ready for storage.
Another thing is the bullets. Some brands will leave more residue than others.