HOT water and soap is great. My preference is Simple Green for cleaning. After everything is clean to my satisfaction, I run a mop with Ballisol inside the barrel and all over the entire gun. I don't like WD-40 or Bore Butter either. Bad experiences with both! I'll never use either one on a gun for long term storage again. I agree with Hawg in that you don't want a petroleum based product. I've had guns stored for YEARS coated with Ballistol and when I got them out, ran a dry patch down the barrel and they were perfectly clean.
“A man who opens a case in hunting camp and lifts out a single-shot BP rifle makes a statement about himself. It may be quite unconscious, or it may be pure egotistical one-upmanship, but it does project the image of a superior marksman and a hunter of above-average skills.”