To add,

Some gas blocks are milled with the gas passage not spaced for a front hand guard ring.  When in doubt, mic the distance of the shoulder to center of the gas port on the barrel, then the same on the gas block to confirm if you need forward off the shoulder off-set.  


With the index line on the barrel shoulder for center of port, and needed amount of off set off the barrel shoulder (if needed) everything should go smoothly as pointed out.  Also, if you do find that you are having gas problem and the gas block index is suspect at a later date, you can always pull the block and check the port fouling burn mark to confirm that the block is indexed correctly (barrel port within the center of the burn mark on the barrel).

Lastly to wrap it up, before installing the block on the barrel, pull the back setscrew and take a look at the alignment of the gas tube hole opening to the block hole.  On some models, you will need to enlarge the gas tube gas entry hole to align with the block passage hole correctly.

P.S. If you do find that you will need to remove the block loctited setscrews at a latter date, best to insert a old allen wrench into the setscrew, then use a torch to heat the wrench it’s self to transfer the head down to break the loctite bond