I am flying with complete AR's (actually Stoner Rifles) all the time because I work for KAC. I can tell you even if you are flying with only half a AR, go ahead and declare it to the desk agent as an unloaded firearm. Then they will walk you over to the x-ray people who will also open the bag and look around. So don't lock your bag yet. Stand there and watch, then hand them the keys after their inspection so they can lock the bag. Not doing it this way may cause your bag to be opened at x-ray without you being present, possible breaking your lock, and or other bad results.
Note, these people are not concerned that "you are only traveling with an inconsequential firearm component". They will not understand. I mean after looking at a couple of tons of dirty underware, they can't wait to open a bag with someting suspicious inside. After all, its their job.