Not sure I have ever seen this happen before but I'm sure it has.   Was shooting with some friends yesterday.  One of my friends is having a little trouble with grouping one of his 5.56 ARs.   We were swapping scopes around to see if that made a difference.

Anyway he was taking his upper down and noticed this on his gas key.










We took some tools and smoothed out the key and shot it some more.  I had assembled that upper for him and as usual I always check, really well I might add, for key clearance.   I always use just the carrier(no bolt) and slowly push the carrier back and forth to ensure the gas tube has minimal friction.

We tried that after we noticed the damage.  It seemed there was a little more resistance than normal with the carrier.  For sanity check we took another carrier and checked and this carrier seemed to have much less resistance with the gas tube end.  I am not sure if the increased resistance we were seeing now was due to the damage or the key might have been slightly off center on the carrier.   This is a rather new carrier(BCG) he was using.

The gas tube was not damaged.   He continued to fire probably another 50 or so rounds and nothing seemed out of the ordinary.  No more damage occurred.

The gas tube appears centered in the upper receiver.   I can get it to move slightly(play) in one direction but not the other.    I am thinking I will take it down and reassemble the upper and center the barrel nut better to give the gas tube a little more play in both directions.   Barrel nut with delta ring assy.

I've had this same issue with my colt upper. It was the first of many problems, i've had with this upper.

Rare, but can happen.

I have seen a match rifle blow a pimer that got lodged next to the gas tube and pushed the tube into the front of the carrier key.

Carrier came forward and stopped out of battery, guy jammed forward assist and bent the gas tube out of the free float and damaged the key like you show.

I'm looking forward to that happening to me. Seems I get all the cool rare shit happening to me.

Did you properly stake the gas key? Was it torqued correctly before staking?

Crooked/bent/misaligned gas tube?

Someone reported running across some gas tubes that were non-spec and not as rigid as normal tubes.
However it seems more likely that something had it pushed out of position.

The gas tube is free float, so if you have a misaligned barrel nut it would equal a misaligned gas tube.  I would take a undamaged BCG, and remove the bolt from it.  Take the CH out from the upper, and put the carrier in.  Turn the upper upside down, and see of the carrier falls out freely or if you have to shake the upper for it to fall out.  If it doesn't fall freely, and tour carrier key is tight, you tube is either bent, out of alignment, or out of spec.

Improperly aligned or bent gas tube.  Is handguard hitting tube and deflecting it to one side or the other?

If the carrier key looks like that, I can only imagine what the end of the gastube looks like.

Replace both.