I rub a nice thick coat of bore butter on the underside of the barrel as a final protectant. Have taken then apart years later and everything looks great.
It may not be PC, but RIG (Rust Inhibiting Grease) is the best and longest lasting rust proofer that I know of. Nobody is going to see it in there anyway.
It is certainly not PC - rather advanced material science stuff. CorrosionX for guns - bonds to the metal and locks humidity out. Not expensive either.