When installing a low-profile LaRue gas block on an LMT M4 profile barrel, I noticed that when aligned with the circle of parkerizing on the bottom of the barrel that was left by the removed FSB the gas block did not index off the barrel shoulder.
Basically I'm lining up the new gas block by centering the hole for the rear set screw on the bottom of the lo-pro gas block with the circle of parkerizing residue on the bottom of the barrel that was left when the barrel was parkerized with the standard FSB in place.
Is this correct?
Here's about all you can do:
1. Make a mark back on top of the barrel shoulder (where the original handguard cap seated) to designate top dead center.
2. Measure the distance from the back of the gas block to the center of the gas hole inside the gas block.
3. Measure the distance from the barrel shoulder to the center of the gas port.
The difference between #2 and #3 measurements will tell you how much gap (if any) to leave between the gas block and the barrel shoulder.
This is exactly what I wound up doing with the addition of adding a tiny (the tip of a 5/32 drill bit) dimple to the barrel for each set screw, and it worked perfectly. Got it all assembled and dumped three mags through it this afternoon and it ran like a top.