I had a mini mauser in 223 that I loaned to a friend for several years. He used sweets on the barrel and did not neutralize it after cleaning. That barrel had etching all thru it and that is the only thing I could think of that could have caused it.
Dirtfarmer mentioned carbon as being about as bad as coppering a barrel. Have two PacNor barrels, one 223 and the other 20 TAC. I use Benchmark in the 223 and 2200 in the TAC. Both barrels seem to sluff off the copper well, but the carbon builds up to where accuracy tails off at about 30-35 rounds. I use Brake Cleaner in the spray can and with a little brushing it comes out and accuracy is back. Even my 204 in 700 Remington carbons after 50 rounds, but has little copper deposits. That is at least I cannot seem to get much out and the accuracy is dynamite. So I cannot have too much coppering.
