I once watched a guy at Minsterly Ranges pick up a bottle of someones Montana xtreme copper killer, and having noticed it does have a bit of an odour proceeded to have a proper old sniff of the bottle, a real lung full. I managed to catch him as he fell backwards, coughing and eyes streaming.
It is fantastic for removing copper and, according to the manufacturer, is totally barrel safe and will not etch. The ordinary Montana xtreme barrel solvent is first class, i use nothing else. I put a squirt of Kroil in it. Don't ask me why, too many American forums been read i suppose.