He's an idiot. Tightening the extension will take it out of alinement with the gas port/FSB. Given only one extension your only option is the same as setting back the barrel on a bolt gun, figure the length needed for the necessary rotation and trim the barrel shoulder and breech face accordingly. At that point you're messing with clearance for the lugs and headspace.
Given a boxful of extensions you'll have a half a day job (assuming you don't get lucky) finding one that is shy of the mark at hand tight.

