I never use bore butter in my barrels. It used to rust my renegade to hell when i'd clean it and then swab it with bore butter. And like you, a week later it was rusty. WD40 is also a horrible oil to use. I'll take a pic of a freshly turned crankshaft that had wd40 applied to it and 2 weeks later, was totally ruined by rust. I no longer oil my barrels. I just dry them, store barrel pointed down and the next day i will run a dry patch, both sides, down the barrel to make sure its fine. No problems yet.