I can't even imagine what happened to your nylon brushes in a single cleaning, mine last for years.
Barnes CR-10 is extremely effective on copper fouling but may be hard on barrel steel too. I would not exceed the recommended max time period to leave that stuff in the bore. Normally, to check for remaining copper fouling just run 2 wet patches down the bore and wait for 2 minutes. Then push a wet patch through and look for the classic blue/green residue. Never use bronze brushes with these aggresive cleaners.
When really stubborn fouling is present I have found it effective to switch between chemical cleaning (CR-10, BBS, Montana Extreme, etc) and mechanical cleaning (JB compound, USP Paste, etc) and then back again if copper is still present.
I have also found "Wipe Out" bore foam to work well and spares you the wear and tear of brushing. It is very effective when used with there "Accelerator".
Just my 2 cents - VH