AR-15 Lower Receiver Parts

Everything that turns a stripped AR-15 lower into a working rifle lives on this page — lower parts kits, fire control groups, buffer systems, grips, and the small springs and detents that vanish into carpet at the worst moments. Impact Guns carries lower receiver parts from Aero Precision, Geissele, Timney, Magpul, and mil-spec suppliers.

Read our full AR-15 Lower Parts Buying Guide ↓

The Lower Parts Kit (LPK): One Box, Complete Lower

A standard LPK contains every component a stripped lower needs: trigger group, hammer, springs, pins, safety selector, bolt catch, magazine catch, takedown pins, trigger guard, and pistol grip. Builders planning an upgraded trigger should buy an LPK without fire control group (sold as “minus FCG”) and skip paying for parts they’ll replace. Mil-spec LPKs from reputable makers are functionally interchangeable; the differences that matter come from upgraded individual components, not kit branding.

Triggers: The Upgrade That Changes Everything

The mil-spec trigger’s gritty 6–8 lb pull is the single biggest accuracy limiter in a factory AR. Drop-in cassette triggers (Timney, Rise) install in minutes with no fitting; two-stage triggers (Geissele SSA, LaRue MBT) give a deliberate take-up then crisp break, favored for precision and duty; single-stage competition triggers break light and fast. See AR-15 triggers for the full category — for most builders, this is where the upgrade budget should go first.

Buffer Systems: Tuning the Cycle

The buffer tube, spring, and buffer weight control how the action cycles. Carbine buffers run H (3.8 oz) to H3 (5.4 oz) — heavier buffers smooth over-gassed rifles and suppressed guns, lighter buffers help under-gassed builds run. Mil-spec diameter tubes are the standard (commercial tubes are slightly larger and limit stock choices). A worn or wrong-weight buffer system is behind a large share of AR reliability complaints; it’s a cheap place to fix a cranky rifle.

Springs, Detents, and the Parts That Fly Away

Takedown detents and their springs are the AR builder’s rite of passage — launched across the room, never found. Spare spring-and-detent kits cost a few dollars and save a stalled build; experienced builders keep one in every parts bin. Magazine catch springs, bolt catch plungers, and safety detents wear over thousands of cycles and are the first suspects when controls get mushy.

Grips and Trigger Guards

Ergonomic upgrades are cheap and personal: Magpul, BCM, and Ergo grips offer different angles and palm swells, and a winter trigger guard adds glove room. These swap in minutes with a single screw and are a sensible place to customize after function is squared away.

Frequently Asked Questions

What parts do I need to complete a stripped AR-15 lower?
A lower parts kit (trigger group, pins, springs, controls, grip), plus a buffer tube assembly (tube, buffer, spring, castle nut, end plate) and a stock. That’s a complete lower — add any mil-spec upper and it’s a rifle. Total assembly takes about an hour with punches and a castle nut wrench.

Are lower parts kits regulated like receivers?
No — every part in an LPK is unregulated hardware that ships directly to your door. Only the serialized stripped lower itself requires an FFL transfer. See AR-15 lower receivers for the regulated half.

What buffer weight should I run?
Standard carbine (3.0 oz) or H (3.8 oz) for most 16” 5.56 rifles; H2 for shorter gas systems and most pistols; H2–H3 for suppressed guns running extra gas. If brass ejects violently forward and the bolt batters, go heavier; if the rifle short-strokes, go lighter or address the gas system.