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Personally I like at least one dimple or flat to be sure it's "locked" in place. Never had one move with or with out them.
Know from a buddy's experience do them after you shoot and verify the gas port is lined up.
Single dimple, and the one closest to the shoulder, which would be opposite the gas port to ensure alignment. If you're absolutely sure that you are not going to change the GB/AGB, then two, but since there are around six different set screw spacings that I know of, it sucks if you change and the forward set screw is slightly misaligned to that dimple. Another exception would be some types of nitride barrels, which can be hard for the set screw to grip without the dimple.
Single dimple opposite gas port, knurled set screw, red Loctite properly applied. That's how SOLGW, BCM and SIONICS do it. Here's Mike of SOLGW explaining the how's and the why's.
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