I've used bore-butter (and my own concoction of neetsfoot oil & beeswax) with no problems at all - when shooting Black Powder! With sub powders I got some slight rust colored residue a couple of times. Not sure if it was actually rust, or just a reaction between the bore-butter and the powder residue, but I switched to Rem-Oil and never had the discoloration again.
Since I don't shoot BP exclusively, I just use Rem-Oil anymore. I do check my bore a few days after I shoot, clean,& store the gun! I then re-oil and have let them set for up to a few months with no problems.