It is my understanding that a permanently attached flash hider on a post-ban AR is illegal, correct? That's assuming that the AR in question has a pistol grip and detachable magazine because you can only have (2) evil features on a post-ban. I mean, if this were legal, every AR manufacturer would have them on their rifles, correct?

you are correct, at least for 129 days  \/

edited to add-- already answered.

Thanks for the confirmation gents. I just wanted to make sure that someone else was incorrect and I was not.

Quoted: It is my understanding that a permanently attached flash hider on a post-ban AR is illegal, correct? That's assuming that the AR in question has a pistol grip and detachable magazine because you can only have (2) evil features on a post-ban. I mean, if this were legal, every AR manufacturer would have them on their rifles, correct?

Not quite correct.  You can only have one evil feature on a post-ban without it being an assault weapon.  In order for a rifle to be in the running as a potential assault weapon it has to be semi-auto and feed from a detachable magazine of some sort.  If it has both of these features then and only then can it be considered an assault weapon if you add any more than one evil feature.  That one feature on 99.9% of post-ban compliant AR-15's is the pistol grip.

As another example a standard SKS with a fixed magazine can have a flash suppressor, bayonet lug, grenade launcher and a folding stock if you so desire.   If you convert it to take detachable AK mags you now have an illegal assault weapon if it still has more than one of the above stated features on it.

Another example would be a standard M1A.  It is semi-automatic and feeds from a detachable magazine but being as their more traditional stock isn't classified as a pistol grip their one evil feature is the flash suppressor.  If however you put a bayo-lug or pistol grip on that puppy then you'll be in trouble.