The U.S. has some weird gun laws. Though we have more gun freedom than any other nation, we still have some odd laws. Take a look at 922 R, suppressor laws, and the way we determine what portion of a gun is a gun. With AR-15s and most handguns, the serialized lower receiver is the actual firearm. That one piece of a gun is considered the firearm. It’s treated no different from a complete AR-15, Glock, 1911, or any other firearm in the eyes of the federal government.
Every other gun part can come straight to your door, but an AR-15 lower requires an FFL. Another unique part of American firearms law is 80% lower receivers.