A copper clad or guilding metal jacketed, when fired, the softer heavier metal, copper, is literally liquified, by the heat ,pressure and friction of the forces involved in the process. It doesn't puddle,it's smeared, I guess might describe it sorta, the length of the barrel. This in essence laps the barrel, making subsequent shots subject to less friction.
I have been reloading for magnum and high powered rifles since 1975. I have observed the fouling shot in numerous rifles. Some barrels foul differently than some others. I have probably spent more time removing it than applying it.
Magnum rifles foul quicker I believe. At least my 300WBY Vanguard fouls pretty quick with warm handloads. The hotter you get your barrel the quicker it will foul. I sometimes shoot quick and cool the barrel every 3 shots with rubbing alcohol, on a rag to the outside, and with a patch on a mop to the inside. This seems to slow it down some and I think the isoprpyl washes a little of the carbon out. Carbon is gritty where copper is smooth, but lead fouling is lumpy, being removed.
As far as removal goes almost all methods suggested will work. I use Barnes CR10. I am reluctant to use abrasives on anything but the roughest bore. I tried it once on 300WBY because I kept getting so much blue for so long. Then I started trying the alcohol thing and it no longer seems to take as long to get better patches.
Anyway, copper removal is payback for our loving to shoot as much as we do. Just gotta get used to it. The same patience goes in to reloading also.