An 80% lower receiver is a portion of a firearm that needs to be completed with 20% work before
it can be used in a functional weapon. Home gun builders use these incomplete components to make
their own firearms from the comfort of their personal workshops.
Rifles and carbines, like the M16, M4, or AR-15, consist of two main systems: the upper receiver
and the lower receiver. A lower receiver (or simply "a lower") typically consists of the grip,
trigger mechanism (bolt, hammer, etc.), and stock of the weapon.