I just threaded two AR barrels last week. They had to be turned down to .499" and then threaded to 1 in 28 thread. I had to set back the flash hiders to time them.
As for the crush washer, if it's done properly (that is the flash hider will tighten down with the solid part at 6 o'clock, it's not really needed. Crush washers were designed for armorers who don't have machine skills. Gunsmiths can either cut the right # of threads or adjust the flash hider such that one is not needed.
BTW, I used crush washers because that's the way ARs are supposed to look.
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