To make your life easier on this one, take a look at Sun Devil Manufacturing out of AZ, they make a patented (I think) carrier key that has an adjustable gas feature, you just replace the carrier key with theirs and adjust as instructed.
http://www.sundevilmfg.com/ADIGS.php
They have videos on their site covering how to install, how it works and how to tune it.
Have several of thier uppers and lowers and really impressed with quality of what they make.
I have bought one to try but have yet to install it yet, not certain when I'll get around to it as current gun runs fine wiht the ammo I use in it.
