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

Complete AR-15 Parts List

Every component you need to build an AR-15 from a stripped lower receiver. We recommend buying a complete upper assembly (covered in Step 3) to skip headspace and gas system complexity, but this list covers both paths. For a specific, vetted recipe with exact picks and a running cost tally, follow the mid-tier AR-15 build.

Step 1: Buying a Stripped Lower Receiver (FFL Transfer)

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. See the best AR-15 lower receivers guide for the nine lowers ranked by stripped, complete, ambi, billet, polymer, and matched-set use cases.

Step 2: Installing the AR-15 Lower Parts Kit & 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). Add an ergonomic grip you like and a stock or brace that locks up tight. Torque the castle nut to spec and stake it; this prevents loosening over time. For help choosing between the two most popular upgrades, see our Geissele SSA vs LaRue MBT-2S trigger comparison.

Start with a carbine H2 buffer for most 16" 5.56 setups, then fine-tune only after live-fire. See the Gas System & Buffer Tuning guide if you need to adjust buffer weight or springs.

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

Step 4: Function Check Your Build

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.

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.

Next moves

After assembly, run a 300–500 round shakedown with quality ammo and mags. Track malfunctions, then adjust buffer weight or springs only if needed, see the Gas System & Buffer Tuning guide for diagnostics.

For the parts that don't come on a factory upper, our ranked picks are in the best AR-15 BCG guide, best AR-15 stocks guide, and best AR-15 slings guide.

When you're ready to add glass, check out the Optic Selection Matrix to find the right optic for your use case, then follow the Optic Mounting Basics guide for proper installation. Planning to run suppressed? Start with the Suppressor Compatibility Basics guide. Building in California, New York, or another feature-restricted state? The parts list above changes: see our featureless AR-15 build guide for the state-specific featureless grip, stock, and muzzle device swaps that clear the assault-weapon feature test.

First AR-15 Build FAQ