New ar upper question

[unlimited4x4]

This is my first AR build. I recieved the upper and after looking it over I noticed this dark red/brown viscous grease like substance where the chamber is and where the bolt lugs would go. What is this? I cleaned it off before going through some function tests with snap caps. Is it left over from the machining process?

 

[thefish]

Machining or shipping grease.
Yes, clean it out.

 

[unlimited4x4]

Thanks, cleaned it. I was just curious as to what it was.