KG-1 for the powder, and then KG-12 for the copper.
After pushing the "initial" crud out the front of the barrel a few times w/ KG-1, soak down the barrel again w/ a wet patch.
Then hit it with a brush just 3-4 swipes to get into the groove corners/break into the hard stuff.
THEN LET IT SIT -- BRUSHED & 'WET' -- for 10 minutes so the solvent can do its real work.
Dry patch out and you're 98% GTG for the next stage of KG-12 copper removal. No brush needed here
Just wet it down, walk away for 10 minutes, and then dry patch out. 99-44/100th's done at this point.
Your real problem may better be described as "layering." The 1st owner probably laid powder over
old copper over old powder over old copper over old powder over copper.... you get the picture.
You can't get at the under layer without removing the top layer -- and this may take several
powder/copper cycles. Weak copper removers will take
forever to eat through a copper layer
to get at the next underlying powder layer. And regardless of what anyone says, Sweets, Hoppes,
Butch's, Shooter'sChoice literally can't cut it so to speak. You'll fill a "...a 5 gallon bucket 1/3 full
of black patches" and still be getting stuff out.
http://www.eabco.com/KG12Test.pdf
http://www.eabco.com/KG12Copper.
Get it at Midway.