I know schools who will not allow homemade AR's in their carbine class. I assume there is a reason.
During the ban, I bought one of the 20" Y2k rifles, without the evil bayo lug and evil flash suppressor. Just before the ban ended, I bought a lower, then after the ban expired, I ordered the patrolmans 16" upper from Bushmaster, and the collapsible stock. No FFL required for the upper. Then I added a bolt, charging handle, etc, and now have 2 complete rifles. It was easier on the wallet that way.
That is one way of building it without being banned from a class. I feel much more comfortable with them, knowing they are built to manufacturer specs.
I think one can be assembled from parts well enough to function. BUT, if you mark up the fire control group area, the Feds can prosecute you of trying to convert it to full auto... supposedly.