Yes, I have seen rust with bh209 before on a couple different guns. They all had one common denominator, they were blued. When I first started using bh209 I had a stainless barrel. I left stainless barrels dirty for awhile (at least a month on one occasion) and did not have rust. So, I thought you could do this with any gun with bh209 and was in for a little surprise when I did this with a blued gun. I told my father in law he could do it too so he got rust in his gun. Lol. This is just my experience and I will only do it with a stainless gun but I still keep an eye on it.