Build Guide
The $400 AR Build That Actually Runs
You can put an AR-15 together for around four hundred bucks. It won't win a carbine match, and it is not a duty gun. It will go bang every time you press the trigger — and for a first build, that is the entire assignment.
First, the honest number
"$400 AR" is a real thing, but it lives and dies on timing. Buy stripped parts on a slow week and you'll clear it. Buy in a panic and you won't. Budget $450–550 all-in once you count shipping, one magazine, and the transfer fee on the lower — anyone quoting you a flat $400 forgot the mag or the FFL.
The trick isn't finding the cheapest parts. It's knowing which cheap parts are exactly as good as the expensive ones (most of them) and which two or three you don't gamble on.
What "runs" means
It feeds, fires, extracts, ejects, and locks back on an empty mag — cold, hot, dirty, every time. That's it. It does not mean sub-MOA, featherweight, or pretty. Reliability is the only spec that matters on build number one. You can chase the rest after you've proven the gun works.
The parts list
The single best decision a first-time builder makes is buying a complete upper — barrel, bolt carrier group, charging handle, and handguard already assembled and headspaced at the factory. It deletes the three steps people actually screw up (headspacing, gas tube alignment, staking the gas key) and turns your "build" into: finish the lower, drop the upper on, function check. Do that first. Piece out an upper on build two.
| Part | Budget pick | Ballpark |
|---|---|---|
| Stripped lower | Any forged mil-spec (PSA, Anderson) | $50–70 |
| Lower parts kit | Standard LPK | $40–60 |
| Buffer tube kit + stock | Carbine mil-spec kit | $40–60 |
| Complete 16" upper (w/ BCG + charging handle) | PSA 5.56 nitride M4, blem/sale | $230–300 |
| Magazine | Magpul PMAG | $10–15 |
| FFL transfer | On the lower only | $25–50 |
Prices are ballparks and they move weekly. The whole point of the deal board is to send you to the live price on the retailer's page — don't take a number in a guide as gospel.
Where to save
These are the parts where "budget" and "premium" are the same aluminum with a different roll mark:
- The lower receiver. A forged 7075 mil-spec lower is a forged 7075 mil-spec lower. The $60 one and the $200 one do the identical job.
- The lower parts kit, stock, and grip. Fit-and-finish, not function. Upgrade the trigger later if you care.
- Handguard finish and muzzle device. A raw or blem handguard runs exactly as well as a pretty one. A plain A2 flash hider is fine forever.
Where you do not save
- The bolt carrier group. Buy one that is MPI/HPT tested with a properly staked gas key. A cheap unstaked BCG will shoot itself loose. This is the #1 reason budget ARs go down.
- The barrel. Nitride chrome-moly is a great cheap barrel — but from a name that publishes its specs, not a no-brand mystery tube. (The complete-upper route handles this for you.)
- Staking the castle nut. It's free. Do it. An unstaked castle nut backs off under recoil and leaves you with a wobbly stock and a dead gun at the worst moment.
Reality check
It'll run a little heavy. The trigger will be gritty mil-spec. The finish won't match. None of that stops it from working, and all of it is a cheap fix down the road. Here's the part nobody selling you upgrades will say out loud: over the life of the rifle, ammo and range time cost more than the gap between budget and premium parts. Build it cheap, prove it runs, and spend the difference on rounds downrange.
Stop reading, start sourcing
The deal board has current BCGs, handguards, PMAGs, and optics — each links straight to the real product page.
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