Shooting copper to clean lead is an old trick.
I abandoned it after thinking about it more closely.
We shoot copper jacketed bullet, with a nicely tapered cone shaped nose, into barrel that has soft metal buildup inside. I never had any problems with this but technically thinking it cannot be good thing to do. That nice taper must force lead residue deeper into grooves as bullet passes by. Eventually lead gets scraped off if we shoot many jacketed bullets.
If we'd have a sharp 90 degree corner on the jacketed bullet which would scrape the lead fouling away, that would be more preferable.
Whenever I have shot lead, I have cleaned it with cleaning rod, some steel wool and patience.
Just yesterday I had a word with my gunsmith friend, and this issue came up. He said one customer of his cracked forcing cone/barrel in a handgun just by doing this copper bullet cleanup thing.
It was a Taurus revolver. Customer cracked one barrel, that was replaced under warranty. Second barrel was not covered by warranty. User abuse.
Gunsmith made him a new barrel from Dan Wesson stock. That has withstood uncracked under abuse.
I've sometimes let chemicals do the barrel conditioning. I have no idea under what names this domestic Finnish stuff is sold abroad, over here it's known as 'Forrest' cleaning foam.
I did a search with this brand and some results came up. I hope these are the same stuff I have here. I recommend this stuff. Seems to work just fine.
(Can looks the same, and the funny applicator as well, but rest of the appearance is different from my can)
http://www.ariantrading.com/forrest.htm
More writing at:
http://www.jacksonrifles.com/maintenance.htm
-Artsi