Well BB did work for me in the same ML I've had since around 1970 and not a speck of rust. It always got a hot soapy water bath and hot water rinse and a coat of BB. But now all my MLs get cleaned with a modern black powder/substitute cleaner and bore conditioner.
Also, as Sabotloader stated for those of you not familiar with metal structure - metals actually have grains much like a piece of granite (only smaller of course). The size of these grains can be controled through different heat treating processes. There is also something called intergranular corrosion which can cause failure of critical components.