gas block question

[LRRPF52]

There are many ways to approach this. Do I focus on a perfect TDC, or acceptable port alignment, but with optimal gas tube to carrier key interface?

I lean to the latter, which is really almost imperceptible when looking at deviation from port alignment between the barrel port and block port. This is one of the most over-looked, yet critical assembly points on the AR15: gas tube to carrier key alignment. If the carrier key clips the tube, you will have accuracy, then reliability problems that will manifest as short-stroking.

Measure your port locations from the barrel shoulder to center of the gas port.

Then measure your port location from the rear of the block to the center of the port inside the block. I use my calipers as a protrusion gauge in this effort, looking through the bottom of the gas block set screw hole or seam if it's a clamp-on type.

Not all barrels are shouldered to the same datum when looking at the ports, because of the old era handguard cap space that needed to be present. Since we basically free-float most of the guns nowadays, there isn't a need for the offset shoulder, especially with the 6.5 Grendel.