Oh not this horse shit again.
There have been numerous threads on this, it has been discussed ad nauseum and then discussed again.
YES it is legal in a semi-auto AR15 when there is no FA FCG present. My new LE6920 has an M16 carrier, and so do most of the Colt LE carbines shipping this year. A post came up some time ago that pointed out a case where the BATFE got burned in court because they could not make an AR15 that had an M16 BCG with semi auto FCG fire on full auto, the case was thrown out.
In no way can M16 carrier alone can not make an AR fire full auto any more than an M16 barrel, handguards or buttstock.
Without the auto-sear, and M16 FCG, the FA Bolt Carrier functions just like a semi auto carrier, except it carries a bit more weight, which is a good thing.