I tried the search function on several topics with no luck.
Are BCM M4 upper receivers moly coated inside?
http://m4carbine.net/showthread.php?t=55930
Snippet from this thread above.
Dry Film Lube
One of the most common questions/concerns/complaints we get on the BCM upper receiver is the interior finish. Some folks think its unfinished anodizing, or worn finish, or some type of problem. What you are seeing is a dry film lube and it’s a good thing. Many commercial upper receivers may skip that step as it is a cost savings, and most folks would not notice the difference. But Uncle Sam wants it on there, and so does BCM. The dry film lube is a grayish milky type finish. It is sprayed on the interior after the hard coat anodizing is applied. It is then baked into the receiver for about 45 minutes at 250 degrees. The dry film lube is just what it sounds like. It adds lubricity to the interior of the receiver where the bolt group and charging handle travel. It’s a good thing.