I just picked up a used Ar upper with about 2k through the barrel.  After a good cleaning I still can't get some stains that look like rust out of the extension.  Used Hoppes, chamber brush, soaked it and flushed with Gun scrubber, nothing.  

I was thinking of using a lead removal cloth when I find one but curious if anyone came across stained extensions before. Near as I can see the chamber is fine just the extension shows staining.  Rifle works fine I'm just anal about a super clean extension after a detailed cleaning.

It's not lead.  It's solid film lubricant, a paint-like coating with lubricant properties.  It is similar to lead in color, it will be spotty and not consistently applied.  It is part of the mil-spec or TDP requirements for the receiver extension.  LEAVE IT ALONE.  It isn't hurting anything and is probably still working as intended, thus reducing friction on the action spring and buffer assembly.

Seldom seen in most manufacturers AR15s these days, and almost never seen in REs bought separately

Stop being anal...it gets in the way of fun and can be harmful to the gun.

I wrote this tired and later realized I didn't fully explain that I was talking about the barrel extension.  There are  stains on the lugs and inside on the extension.  Maybe cause it came from a high humidity area some rust did form although breach face of the barrel and chamber are spotless.

Anywho since this is a new to me upper I wanted it spotless as a better way of looking it over and make sure there wasn't something that needed fixing or replacing.  Will order a new barrel to have on hand when this stops shooting moa.

I would leave it, but maybe something like rem action cleaner a drill and a chamber brush.

you are going to have to polish those stains out.

Wow you are anal, OCD or just plain nuts. I thought I was bad with keeping things clean and nice.

Clean it good and if there are stains it is not an issue. The stains could be discoloring from heat treating. I have noticed on some of the barrels I bought the extensions are discolored from heat treating.

Can you post pics?

I always have problems with photobucket so I don't post much pictures.

Anywho I've just never seen staining like this out of my 3 other ARs not to mention all the M4s in our station armory, drives me nuts but I'm not nuts.  I'm sure I'll get used to it if I can't get it off.