I am no expert on cleaning methods or barrel metalergy, but have picked up a few things through the years.
Depending on the barrel, all will pick up copper from shooting no matter whos, or how polished they are. The degree of fouling could be determined by several factors. If the bore is rough it will foul quicker than a smooth one. Depending on the jacket material used on the bullets, it may foul worse with one brand than another, as well as the speed at which your driving the same bullets.
When using the Sweets, you need to be really sure you give the barrel a few extra patches of standard solvent after which you should follow up with at least a couple of dry and finially one oiled patch. This will ensure that you get the Sweets out and not cause more problems than you need. I use Sweets on a limited basis, mainly as an indicator. It is really strong and will definately let you know if there is any copper left in a tube. IT only takes one drop on a patch with an eye dropper to determin wether or not you need more scrubbing. IF I use it I always follow the proceedure above to make sure I get the Sweets out.
There are some really good cleaners out there now, as well as some which claim to be. Personally I use three for 99% of all my cleaning. Butch's Bore Shine is probably the top one I use, it is pretty good at hitting the carbon and gets the copper out decent as well. A 2-1 mixture of Shooters Choice and Kroil is almost identical if not so to the quickness and thoroughness of the Butch's, and I use it on several older rifles, simply from habit more than anything. Lastly I use Hoppe's when I am in the country as I have the big bottle there for cleaning in case of rain or any other situation that arises when I leave my other stuff at the house.
As I mentioned , I also have Sweets on hand and when it is used on a barrel it is in very limited amounts. I did have a 243 I clened up the other day which was a used rifle when I picked it up. It has just been sitting waiting on the grandson to get big enough to shoot it. After about an hour of working on it with the previous mentioned methods, I went extreme on it and broke out the Sweets. I gave it a complete scrubbing several times over and after getting a clean patch out with the Sweets, I went back through it with the Butch's and several coats of oil to ensure I got the sweets out. I figure that the little thing had so much fouling built up in it the previous owners figured it was shot out or wasn't worth the effort. Here are the initial 3 - 100yd bore sight shots after the process, with factory ammo,
Hopefully I can get my handloads to do as well.