I find the barrel will get very tiny dots of rust if it gets wet and I don't have it oiled. The dots will not spread, but they are present. I keep it lightly oiled with CLP or such and it is fine after that.
One of my hunting partners Krylons his barrel which works fine and his is a hard working rifle. In theory it could be ceracoated but would require bead blasting off the nitride coating which I suspect would be a real pain.
I don't know if they coat the inside of the barrel. I have heard different stories and can't confirm anything. The barrels though wear very well.
I like Phil Shoemaker's approach to corrosion resistance where he says:
"The one problem I have with making the outside of a rifle too weather proof is that you then don't always recognize what is happening inside the bore.
A rusty outside acts like an idiot light and tells you that something that counts inside is likely to need your attention."
I have had stainless rifles develop rust in very wet rainforest conditions (Fiordland, NZ - Winchester M70 Extreme SS). I just treat all rifles as if they will rust and maintain accordingly.