Building Your First AR-15: The Parts You Actually Need

The short version

An AR-15 is two halves: the lower receiver and the upper receiver. Each half needs a handful of parts, plus a few small kits that are easy to forget. The AR-15 Studio adds those kits for you when a part needs them.

The lower

The stripped lower receiver is the serialized part, so under federal law it is the firearm. You buy it through a licensed dealer with a background check, the same as a complete gun.

A forged mil-spec lower like the Aero Precision M4E1 (about $155) is a solid start. It also needs:

A lower parts kit: springs, pins, safety, magazine catch and the fire control group, about $55.
A trigger, if you want better than the kit's. The BCM PNT (about $66) is a polished mil-spec style trigger that is easy to install.
A buffer tube kit: tube, buffer and spring, about $60 for carbine length.
A stock and a grip. Most stocks are sold for a mil-spec carbine tube.

The upper

The upper holds the barrel and the bolt. Buying it assembled, with the forward assist and dust cover already installed, saves time and special tools. The Aero Precision M4E1 Threaded Assembled Upper (about $160) is a good example.

Then you need:

A barrel. A 16 in 5.56 mid-length like the Ballistic Advantage Modern Series (about $175) is a classic first choice.
A gas block and gas tube if the barrel doesn't include them, about $45.
A bolt carrier group. One standard BCG works for both 5.56 and .300 Blackout. Other calibers, like 6.5 Grendel, need their own bolt.
A handguard. A free-float M-LOK rail such as the Aero ATLAS S-ONE 12.7 in (about $175) leaves room for a light and a grip.
A charging handle. The BCM Gunfighter (about $80) is a popular upgrade over basic mil-spec.
A muzzle device. Many barrels come with an A2 birdcage.

Barrel length and the law

On a rifle, a barrel under 16 in makes it a short-barreled rifle (SBR) under the NFA. A 14.5 in barrel only counts as 16 in if the muzzle device is permanently pinned and welded to reach that length. Barrels at 16 in or longer avoid the question.

In October 2026 ATF said it would stop enforcing NFA registration for SBRs and suppressors after federal court rulings. That is a policy, not a repeal, and it could change. Many states have their own rules, and taking an SBR across state lines still needs ATF Form 5320.20. Check current federal law and your state and local law before you buy or install anything, and ask a firearms attorney if you are unsure.

Magazines and an optic

A rifle you can't feed isn't finished. Magpul PMAG 30 GEN M3 magazines are about $16 each. For an optic, a SIG Sauer Romeo5 Gen II red dot (about $129) comes with its own Picatinny mount and risers, so it bolts straight onto a flat-top upper.

Tools worth buying

Follow the torque specs from each part's maker. If you're unsure about headspace on a new barrel and bolt, a gunsmith can check it with gauges for a small fee.

An AR armorer's wrench and a torque wrench for the barrel nut and buffer tube castle nut.
Roll pin punches and a small hammer for the lower parts kit.
Upper and lower vise blocks so you don't clamp the receivers directly.

What a basic build costs

With the parts above, a first build lands around $1,000 to $1,200 before ammo and optic upgrades. Cheaper parts exist and so do much nicer ones. The studio totals everything as you pick, including the small kits.

Prices are rough street prices and change often, so check current listings before you buy.

Parts in this guide

Aero Precision M4E1 Stripped Lower Receiver

$155 · Lower receiver

BCM PNT Trigger

$66 · Trigger

Aero Precision M4E1 Threaded Assembled Upper Receiver

$160 · Upper receiver

Ballistic Advantage Modern Series 16" 5.56 Government Profile Mid-Length Barrel (BABL556015M)

$175 · Barrel

Aero Precision ATLAS S-ONE M-LOK Handguard 12.7"

$175 · Handguard

BCM Gunfighter Charging Handle

$80 · Charging handle

Magpul PMAG 30 AR/M4 GEN M3

$15.95 · Magazine

SIG Sauer Romeo5 Gen II

$129 · Optic or sights

Open the AR-15 builder

Prices are typical and change often. This guide is general information, not legal advice. Suppressors and short-barreled rifles are NFA items; check federal, state and local law.