Aren’t chrome-lined bores supposed to mitigate this to some extent?
Yup. Key part being “some extent” 
I’ve gotten spots of rust on my gas port (chromed), gas cylinder (chromed), and inside the muzzle brake (chromed), in addition to gas tube (non-chromed).
Hmm, I checked my muzzle break and noticed some rusting. Guess I gotta clean it more thoroughly.
Carbine ammo was never corrosive. 