I have 4ARs and 1GB. I found that they have different liner thickness. Some have liner thickness of 2.0mm, while others have thickness of 2.5mm. Why? the 2.0mm liners are stonewashed, the 2.5 mm liners are not.
http://www.knifriend.com/bbs/viewthread.php?tid=71877&fpage=1&highlight=
http://www.knifriend.com/bbs/viewthread.php?tid=71877&fpage=1&highlight=