Exterior of gas tube coated in carbon

[higgens]

While changing an ALG defense rail, I noticed the exterior of the gas tube is coated in carbon for 3" beyond the upper (for the full barrel nut length + 1") and its coated 1" at the gas block. This is for a 16" midlength barrel. It has maybe 1000 rounds through it with this rail, no malfunctions. I normally don't change out rails, is all this carbon expected or a sign of a problem?

[17K]

Normal.

[Coal Dragger]

Totally normal.

[tom12.7]

As long as it's an accumulation of thinner soot layers, then yes it's okay. Heavy streaked lines can be another story.