I recommend 680 AS AN OPTION also,
On my first upper build....
Had some left over in my gun box. Wiped a small amount on mating surface. Then assembled. Just be careful not overuse or to put it right up against the port hole, as it does seek air, and expand as it sets.
I used an off brand gas key no lock tight on the threads, and did not have an inch pound torque wrench, just went by feel (ye I know,
), but still no leaks to this day.