What Is in an AR-15 Lower Parts Kit and What Do the Parts Do?
An AR-15 lower parts kit (LPK) contains all the small parts needed to complete a stripped lower receiver: trigger, hammer, disconnector, trigger and hammer springs, trigger and hammer pins, safety selector, detent and springs for the safety and buffer retainer, magazine catch and button, bolt catch, pivot and takedown pins with detents and springs, pistol grip with screw and washer, and the trigger guard. A complete LPK has approximately 30 individual parts.
Why It Matters
The lower parts kit turns a stripped lower receiver into a functional firearm. Understanding what each part does helps you build confidently, troubleshoot problems, and know which parts are worth upgrading and which are perfectly fine as mil-spec. Many builders buy a complete LPK and then separately upgrade the trigger and grip, which is the most cost-effective approach.
The Detail
Lower parts kit components by function group:
Fire control group (the trigger assembly):
- Trigger: the lever you pull to fire the rifle
- Hammer: strikes the firing pin when released by the trigger
- Disconnector: catches the hammer after firing so it does not follow the bolt forward
- Trigger spring: returns the trigger forward after release
- Hammer spring: drives the hammer forward to strike the firing pin
- Trigger pin: holds the trigger in the receiver
- Hammer pin: holds the hammer in the receiver
- NOTE: upgrade this entire group with a drop-in trigger for the biggest improvement
Safety and controls:
- Safety selector: rotates between safe and fire positions
- Safety selector detent: spring-loaded ball that holds the safety in position
- Safety selector detent spring: provides tension on the detent
- Magazine catch: the button and arm that hold the magazine in place
- Magazine catch button: pressed to release the magazine
- Magazine catch spring: provides tension on the magazine catch
- Bolt catch: holds the bolt open on an empty magazine
- Bolt catch roll pin: secures the bolt catch to the receiver
- Bolt catch spring: returns the bolt catch to its resting position
- Bolt catch plunger: actuated by the magazine follower
Pins and retention:
- Pivot pin (front): connects upper and lower receivers at the front
- Takedown pin (rear): connects upper and lower at the rear, pulled to separate
- Pivot pin detent and spring: holds the pivot pin in place
- Takedown pin detent and spring: holds the takedown pin in place
- Buffer retainer and spring: holds the buffer in the buffer tube (small but important)
Grip and trigger guard:
- Pistol grip: A2 grip standard in most LPKs (usually the first thing replaced)
- Pistol grip screw and lock washer: secures grip to receiver
- Trigger guard: protects the trigger from accidental activation
- Trigger guard roll pin: secures the trigger guard (some lowers use integrated guards)
Parts worth upgrading from mil-spec:
1. Trigger: biggest accuracy improvement (LaRue MBT-2S, Geissele SSA-E)
2. Pistol grip: ergonomics matter (Magpul MOE+, BCM Gunfighter, B5 Systems)
3. Safety selector: ambidextrous for better manipulation (Radian Talon, BAD-ASS)
4. Magazine catch: extended for faster reloads (Magpul, Seekins)
Parts that are fine as mil-spec:
- All pins and detents (mil-spec is the standard)
- Bolt catch (mil-spec works perfectly)
- Buffer retainer (mil-spec is universal)
- Trigger guard (unless you want winter use, then oversized)
Build Impact
For a first build, buy a quality complete LPK from Aero Precision, CMMG, or BCM for $30 to $50, then separately buy an upgraded trigger (LaRue MBT-2S for $100) and pistol grip (Magpul MOE+ for $20). This gives you a properly functioning lower with the two most impactful upgrades for approximately $170 total. Watch a YouTube assembly video before starting — the detent springs are notoriously tricky and like to launch across the room.