In the US (not sure about your location or the laws in other countries), the part of an AR-15 that qualifies as the "firearm" in the eyes of the law is the lower receiver. It's a machined block of metal and does not do anything on its own.

When transporting a firearm through air travel, it's supposed to be locked such that only the owner can open it. So you can claim you're carrying a firearm with nothing but a small-ish piece of metal in your luggage.

For illustration: https://i.imgur.com/ARZgZV6.png. The non-transparent part is the lower receiver.

As an aside, the "AR" in AR-15 stands for Armalite Rifle (after the company that originally designed it); not "assault rifle".