While cleaning my AR today I noticed that I seem to have a mark/scoring in the barrel about halfway down on one of the lands.  It is a chrome lined barrel with about 1,000 rounds through it.  I don't know what could have caused this.  Has anyone else ever seen this?  For what its worth, it is a Bushmaster XM15 shorty.  

Without seeing the thing first hand, I can only give you two educated guess possibilities:

1)Something got into the barrel that left the scratch.  Since I assume it's a chrome lined piece, the scratch would most likely be surface only, and not affect accuracy

2) It's an imperfection in the chrome lining itself, and might be the start of the lining peeling at that point.  Very rare, but it does happen.

Keep an eye on it for awhile. If it tends to fade somewhat as you use the piece, it's probably 1).  If it gets more noticable and larger as use fire more rounds, you may be looking at 2).  If so, I'd contact Bushie about having them take a look at it.

Is it the gasport?