How to Build an AR-15: Complete Parts List & Step-by-Step Guide (2026)

Step 1: Buy a stripped lower receiver

The lower receiver is the regulated part. Order a stripped lower online from a reputable manufacturer and ship it to your local FFL for transfer, or buy one directly at the shop. Bring ID, complete the 4473, and keep the receipt with your serial number for records.

Avoid bargain-bin or polymer lowers (unless it's a reinforced KP-15). Stick with forged 7075-T6 from brands like Aero, BCM, Geissele, or PSA. Check that the takedown pins move smoothly before you leave the counter.

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Forged, billet, complete, ambi, polymer, matched set. Nine AR-15 lower receivers ranked by buyer use case, with FFL realities, M16-pocket SBR notes, and the trigger compatibility gotchas no one warns you about.

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Step 2: Install the lower parts kit and trigger

Tools needed: Armorer's wrench, punch set, torque wrench (in-lb and ft-lb), vise block, and a clean workspace. See our AR Tools Guide for the complete list and recommendations. Torque specs and threadlocker guidance are in the reference section below.

A solid lower parts kit gives you reliable springs, detents, and controls. Avoid no-name kits; pick PSA, Aero, BCM, SOLGW, or Forward Controls–sourced components. Reference the Lower Parts Quality Guide for vetted options.

Select the trigger that matches your mission (mil-spec for duty durability, or a known single-stage/2-stage upgrade like ALG ACT, Geissele SSA, or LaRue MBT). Install the trigger and hammer with their pins, then the selector, and finish with the bolt catch and magazine catch. The 3D view slides those controls out of the receiver so you can see where each one seats. For help choosing between the two most popular upgrades, see our Geissele SSA vs LaRue MBT-2S trigger comparison.

For a step-by-step visual walkthrough, check out Pew Pew Tactical's lower assembly guide, it has detailed photos for every step.

Best AR-15 Lower Parts Kits 2026: Top Picks Ranked (SOLGW, DD, CMMG, Aero)

Best AR-15 lower parts kits ranked for 2026: SOLGW Liberty best overall, Daniel Defense best complete, CMMG best budget, Aero best for trigger upgraders, CMC for a kit with the drop-in trigger already in the box. Comparison table, upgrade priorities, and LPK cost calculator.

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Best AR-15 Triggers 2026: Geissele vs LaRue vs ALG Compared

Best AR-15 triggers ranked: LaRue MBT-2S best value, ALG ACT budget king, Geissele SSA duty proven. Honest assessment of diminishing returns and why reset speed matters more than break quality.

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AR-15 Build Tools 2026: Complete Kit List ($100 Budget to $500 Pro Setup)

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Step 3: Grip, buffer tube, and stock

Three parts turn the finished lower into something you can shoulder. Start with the pistol grip: drop the selector detent spring into its hole in the grip, then drive the grip screw and lock washer to 25-35 in-lbs. If the selector will not click between SAFE and FIRE afterward, the detent spring is pinched or missing.

Next the buffer tube. Thread the castle nut and end plate onto the tube, screw the tube into the receiver until it captures the buffer retainer, then torque the castle nut to 38-42 ft-lbs and stake it into the end plate. Staking is what keeps it tight; an unstaked castle nut is the most common way a first build loosens up. Drop in the spring and buffer: a carbine H2 buffer is the right start for most 16" 5.56 setups, and the Gas System & Buffer Tuning guide covers adjusting weight or springs only after live fire.

Last, slide a mil-spec stock onto the tube (rifle build) or a brace (pistol build). Pick one that locks up without rattle; a Magpul MOE SL or CTR is the safe first choice. Which of the two you can legally fit depends on the barrel length you choose in the next step, so read the NFA note below before buying a brace.

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Step 4: Choose and attach a complete upper

A factory complete upper is the whole top half of the rifle in one box: receiver, barrel, gas block and tube, handguard, and muzzle device already assembled, headspaced, and gas-timed. That is the work a first-time builder should not be doing with a vise and a torque wrench. Pick the tier that matches your budget: PSA (entry), Aero Precision (mid-range), BCM (duty-grade), or Geissele (top tier). All four run reliably when the lower is built right, and the complete upper receiver guide below ranks the specific SKUs.

Read the listing carefully for two words: bolt carrier group and charging handle. Many uppers ship without one or both, and a "complete" upper with no BCG will not fire. The PSA 16" nitride mid-length upper in the reference build below includes both; if yours does not, buy them from the same tier as the upper. The best AR-15 BCG guide and charging handle guide rank the options.

A 16" barrel with a mid-length gas system is the right first upper: it keeps the rifle out of NFA territory with a stock, cycles softer than carbine gas, and shoots every common 5.56 load. The barrel length guide breaks down velocity, dwell time, and handling from 10.5" to 20" if you want to go shorter or longer.

Mating the halves takes ten seconds: push the pivot pin (the front one) out, hook the upper's front lug into the lower, push the pivot pin through, close the upper, and push the takedown pin home. Both pins should move with thumb pressure and stay put. Building the upper half yourself instead? The stripped upper receiver guide starts that path, and the full parts checklist further down lists everything a from-scratch upper needs.

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Step 5: Function check

Before loading any ammunition, verify your rifle functions correctly. Clear the weapon visually and physically: lock the bolt back and confirm the chamber is empty. The rifle in the strip runs the check for you on this step: the charging handle and bolt carrier go through a full stroke to cock the hammer, the trigger is pressed, and the hammer falls, with the controls you are about to test lit and the rest of the rifle faded.

Warning: Never drop the hammer on your lower receiver without the upper installed. The hammer striking the bare receiver can crack the aluminum over time. Always complete your upper/lower assembly before testing trigger function.

1. Trigger & hammer reset: With the safety OFF, pull the trigger and hold it. Manually cycle the charging handle to reset the hammer. Release the trigger slowly, you should feel and hear a click as the disconnector releases the hammer to the trigger sear.
2. Safety selector: Cock the hammer (cycle the charging handle). Flip the safety to SAFE. Pull the trigger, nothing should happen. Flip to FIRE and pull, the hammer should fall.
3. Bolt catch: Lock the bolt back using the bolt catch. It should hold firmly. Press the bolt release, the bolt should snap forward cleanly.
4. Magazine insertion: Insert an empty magazine. It should lock in with a solid click. Press the magazine release, it should drop free without sticking.

If any step fails, diagnose before live fire. Common issues include improperly installed safety detent springs, trigger pin walk, or a misaligned bolt catch. Double-check your lower parts installation.

Once every check passes, oil the bolt carrier group (bolt lugs, cam pin, carrier rails, and gas rings), load quality current production magazines from Magpul or DURAMAG, and head to the range for a zero and a 300-500 round shakedown.

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