It's safe to leave Hoppe's #9 in your barrel overnight BUT it wouldn't make sense to me to do it because what if it leaks onto your stock, carpet, etc? Also it wouldn't do as good of a job as using AGRESSIVE copper cleaner such as Sweet's 7.62 as directed. If you want to be able to clean your barrel the lazy way, go with the Outer's Foul Out or Foul Out 2. It DOES work.