The Colt SPK99501 is a comprehensive, genuine Original Equipment Manufacturer (OEM) lower parts kit (LPK) tailored for semi-automatic AR-15 and M4 platform variants. Assembled by the primary developer of the original AR platform, this kit features everything needed to populate a stripped lower receiver or complete an overhaul on a high-mileage duty rifle. Every component is produced to small-pin (.154" diameter fire control pins) mil-spec tolerances out of premium heat-treated steel and durable polymers, preventing the tolerance stacking issues common to mixed-brand kits. The inclusion of a full single-stage trigger group, standard A2-style pistol grip, and necessary springs ensures immediate drop-in compatibility, providing smooth control mechanics and battlefield-grade reliability. Colt 223/5.56mm AR Lower Receiver Parts Kit (Semi) SPK99501