The shim kit is used between the cylinder and the barrel shoulder. This shimming just tightens up the cylinder on the barrel to keep it from moving (read destroying barrel harmonics), and to keep the now lock/front sight from moving around on the barrel.
Read: Shim the cylinder forward on the barrel to have the lock/sight go hand tight at the 3 o’clock position, then use the lock wrench to take it to the 6.
As for the gas plug, the torque spec for the gas plug is 15 lbs, but the AMU guys are tightening there’s to 21 LBS (cold). Just remember to back up the lock when you tighten the plug so you don't transfer torque threw the cylinder on the barrel.
Mach 6, the sight plain focal distances is what I was referring to, and not the effects of the items added to the barrels. Simple put, the father the front sight is to the rear sight, the greater you will see your deviations in your POA. If the unit is going to get optics, then the iron sight plain is just moot.