I have noticed premature wear on a couple of my AR's gas tubes after 2000 rounds. They were properly fitted, but why the excessive wear? O.K, so I take a close look at the inside of the gas keys. No surprise, they have the appearance of a fine file. Yes, they are chrome lined, but rough. One more thing to add to the checklist when building or replacing parts. Anyone else experience this?

I have noticed and rejected/replaced a couple rough carrier keys.  I have not seen a functional problem I could attribute to them however.

Like you, I figure it can't be good for the gas tube.

In my first post, I said premature wear. Let me change that to excessive wear. The end of the gas tube was worn completely slick. It was a little weak on cycling, but functioned ok with heavy bullets. The roughness did not cause any function problems.