I also had rust problems using Bore Butter in the bore as a rust preventitive, after cleaning, before storing. I only use Bore Butter as a lube for Maxi-Balls, and sometimes as a lube for patches for round balls. I use Goex Black Powder, and use Clenzoil, in the bore to keep rust out after cleaning, in all my ML's. No problem with fouling, so I don't agree with the "use no petroleum products" myth. IMHO some petro products will cause fouling, but not all.