Have a look on this site for information on Chrome plating:
http://www.electroplating.co.uk/chrome.htm
A note from that site on corrossion resistance may answer why barrels should be cleaned and oiled:
Note on Hard Chrome
It is a common misconception that chrome plating provides corrosion protection. This is not the case, only slight protection is given. Hard Chrome is porous at the microscopic level, which can let moisture and chemicals through to attack the base material. If the item is to be used in a damp environment, or where water based inks are used on printing machines, a layer of Nickel plating is required, prior to hard chrome plating.
Paul