patch with lighter fluid on it, negates any issues.
Test: any product on carbon build up on muzzle breaks, inside, and on crown of rifle.
If you love a good penetrating oil, then you should try Free All...really amazing stuff, I have tried these:
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If you are so inclined to make your cleaning process a LOT easier, push 4 patches of Free All down the bore when you are through shooting. Kroil will help, but Free All is like Star Trek Technology. I use a sandwich bag over the muzzle with rubber band attachment to keep the fluid out of the gun case.
Carbon build up is a serious enemy, bitch to get out depending on the powder type.