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AR-15 Lower Parts Kits: Complete Guide for Builders and Enthusiasts When building or upgrading your AR-15, choosing the right AR-15 lower parts kit  (LPK) is critical to ensure optimal performance and reliability. The LPK consists of essential components that complete your lower receiver, such as the trigger assembly, safety selector, bolt catch, magazine catch, takedown pins, and various springs and detents. These small yet vital parts work together to facilitate the p …