I have taken my AR556 out in the very early morning humidity here in Louisiana, just to get very small patches of surface rust on the barrel. And, I am very anal about keeping the whole gun lubed with Ballistol or Hoppes gun oil. I have built 7 AR's prior to owning my AR556, so I do have the tools. And, am very familiar with the AR556's barrel nut. I have replaced OTHER AR556 barrels for friends.
A company in Ohio will Nitride my barrel, inside and out, for $85. And, they pay return shipping.