I can tell you for a fact that the MR556 barrel is not nitrided.
If it had been, my adventures in barrel profiling and cutting it back to 14.5 to pin and weld the flash hider on would have been far, far different. I would have had a post in here about having to buy new bits for the lathe.
Nitriding is a fantastic process and I am very much looking forward to having my barrel QPQ nitrided this month. Insofar as i'm concerned, nitriding will make my barrel superior to a 416 barrel in all aspects once it has been nitrided.
I, for one, am very glad HK did not nitride the MR556 barrels. By not doing so, they made it so that we (people who like to Mod HK import guns) can have the exact barrel profile we want, length we want (16.5 inches and less), and add then nitride.
As far as i'm concerned, this is proof to me that HK actually is paying attention to the US civilian market and while they know that they are hobbled by US import restrictions to release neutered, crappy versions of their good guns, they at least give us the raw materials to make versions that are closer to what we want, whatever that want may be.
To be honest, if HK had factory nitrided the barrels, I wouldn't have bought an MR556 to begin with. I'd have a scar or something else in my stable instead.
For what it's worth, thank you HK for shipping these things to us in the raw like you did. Makes project gunning much, much easier.