A lower receiver is a crucial component of a firearm, specifically in AR-15 style rifles. It houses the trigger mechanism and is the part that connects to the stock and grip. The lower receiver is often considered the firearm's "firearm" under U.S. law, meaning it requires a serial number and must be registered. In addition to its legal significance, the lower receiver is where various parts, such as the trigger, hammer, and magazine release, are installed. It plays a vital role in the overall function of the rifle, allowing for the assembly of other components like the upper receiver and barrel.