Daryl's wet patching method works.
I was shooting today, and followed Daryl's recommendation for firing a reduced load for the last shot, wet-lubed as usual. After the last shot, I ran a breechface scraper down. It came back with nothing. As Daryl said, the wet patching pushes the fowling down on TOP of the powder charge, and all of that goes out the muzzle on firing.
You are only cleaning out the fouling from the last shot, not building up a fouling pile at the breech to get built up with each shot and hardened. The reduced charge last shot simply makes the cleaning easier.
For a load that'll be in a bore all day, like hunting, I'd go for something like mink oil to not foul the charge over time.
Best wishes, Marc