Kroil is a penetrating oil. As the can states "The oil that creeps" (or is it the Creep that oils?
). Anyway, somewhere, someone discoverd that the stuff removes stubborn powder residue from rifle barrels. It doesn't do anything for copper, but clears out powder in a hurry. The procedure is to brush then a couple of patches with Kroil to clean out the powder then your favorite bore cleaner can do it's job on the copper. Another is brush,wet the bore with Kroil followed with JB. One of the favorites is to mix 50-50 Kroil and Shooter's Choice and get both at the same time. You need to watch what you mix with bore cleaners. Oil has a tendency to degrade the ability of some cleaners to desolve copper. I use TM bore cleaner from Sinclair and don't mix them.
I was a maintinence tech for 26 years. During that time I hve no idea how many gallons I used for everything from freeing stuck parts to sharpening knives. In the mid 70s, the late Bob Pease put me onto it as a bore cleaner and have used it ever since. Now that I'm retired, I gotta buy the stuff...BUMMER!
A few others are used such as GM Top engine cleaner and Mystery Marvel oil, but Kroil seems to be the top choice.
The best advice I have is to get a can, give it a shot and decide for yourself. If you do any mechanical work, I'm sure you'l find another 1000 or so uses for it. Midway,Sinclair have it and if you search a bit, maybe some local hardware stores might carry it.
Here is a bit of reading material .........
6 BR cleaning