Aero Precision stuff is of very good quality in my experience. Their forgings are very clean and their fit/finish is right up there with some of the best in my experience with them.
It is rumored, not confirmed because of various confidential business relationships/agreements, that they make receivers (upper and lower) for major players like Armalite, Spike's, and for tons of different "small shop" AR retailers who want to put their name/rollmark on a lower and sell them as their own. Again, I have never seen any documentation that will confirm/refute such claims as to whom they do/don't make parts for, but they are one of the bigger players in the game for sure when they aren't cranking out parts/equip. for the aerospace industry (I think they have, at least in the past, done a ton of work for Boeing...not ARs, but precision equipment nonetheless).