While the lower receiver is the foundation of the firearm, the upper receiver houses the components that most largely determine the performance of the rifle: the bolt carrier group and the barrel. Good news is, unlike a lower receiver, uppers are not considered firearms by the ATF! Meaning you can purchase the upper and have it shipped straight to your door, no FFL transfer and no paperwork. Also, you can assemble a variety of upper configurations and use most with the same lower, meaning you only have one serialized firearm capable of shooting a variety of calibers.
