Lower receivers are a crucial component of firearms, specifically in AR-15 style rifles. They house the trigger mechanism and are the part of the firearm that the magazine feeds into. 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 their legal significance, lower receivers come in various designs and materials, affecting the overall weight and performance of the rifle. Many enthusiasts customize their lower receivers with different grips, stocks, and accessories to enhance functionality and comfort during use.