Stripped Lower Receiver: How to Choose the Right One
The stripped lower receiver is the part the government calls the firearm, so it is the one piece of an AR build you choose with care.
The first part you order is the only one the government cares about. Everything else in the box is hardware. As a Type 07 manufacturer and a gunsmith of more than twenty years, I have watched a lot of builds start right here, and the ones that go wrong usually went wrong on this one purchase. Get it right and the rest is assembly. Get it wrong and you will feel it every time you drop a magazine.
So what is it? A stripped lower receiver is the bare, serialized housing that holds the fire control group, the magazine well, and the buffer tube on an AR-pattern rifle. It ships with no trigger, no springs, no pins, and no stock. Under federal law it IS the firearm, which means it transfers through an FFL with a background check exactly like a complete rifle.
Forged, Billet, or Polymer?
Three manufacturing methods dominate the market, and the differences are real without being dramatic. Any of the three will run. Let’s line them up.
| Type | How it is made | Strengths | Trade-offs |
|---|---|---|---|
| Forged | 7075-T6 aluminum pressed into shape, then machined | Densest grain structure, strongest, least expensive | Plain profile, limited feature options |
| Billet | Cut from a solid 7075 or 6061 block | Integrated trigger guard, flared magwell, tensioning screws | Costs more, sometimes proprietary upper fit |
| Polymer | Reinforced polymer, often with metal inserts | Lightest, corrosion-proof, cheapest | Less tolerant of hard use and over-torqued fasteners |
For a first rifle, forged is the answer. A forged receiver from a reputable maker is the same alloy and heat treatment used on service rifles. Builders pick billet for the flared magwell and the integrated trigger guard, and those are comfort and speed features (not durability upgrades). Polymer earns its place on a light-use plinker or a rifle that lives in a boat.
Five Things to Check Before You Buy
Five things separate a receiver you will keep from one you will replace.
- Alloy and finish. Look for 7075-T6 with a Type III hardcoat anodized finish. 6061 is softer. On a billet receiver it is acceptable; on a forged one it is a shortcut.
- Threading quality. The buffer tube threads and the pistol grip screw hole are where cheap receivers fail. Cross-threading here ends the build.
- Pin hole alignment. Trigger and hammer pin holes out of spec produce a gritty trigger that no amount of polishing fixes.
- Magwell finish. Run a magazine in and out ten times. Drag on a bare receiver only gets worse once the rifle is dirty.
- Roll mark and serial. Confirm the serial is legibly and permanently marked. You will need it for every transfer for the rest of the rifle’s life.
The Mil-Spec Dimensions That Decide Fit
“Mil-spec” on a receiver means the critical dimensions match the technical data package the military uses, so parts from different makers interchange. That interchangeability is the whole point of the platform.
The dimensions that matter most are the takedown and pivot pin diameters at 0.250 inch, the trigger and hammer pin holes at 0.154 inch, and the buffer tube thread at 1.185 inches by 16 threads per inch. Remember, a few receivers are cut for the old Colt-pattern 0.170 inch large pins, and those will not interchange with standard 0.154 inch parts. Confirm the pin size before you order a trigger.
Makers publish these numbers. Rock River Arms and Aero Precision both list receiver specifications on their product pages, and checking the spec sheet takes less time than a return shipment. I covered one of Rock River’s matched sets in my look at the BB0301 upper receiver, which is the other half of this equation.
The Paperwork Side
Because the receiver is legally the firearm, it ships to an FFL, not to your door. You fill out a Form 4473 and pass a background check to take it home, the same as buying a complete rifle. The definition lives in 27 C.F.R. § 478.12, and it is worth reading once.
Now, this is also where a receiver differs from an unfinished blank. That distinction has been in front of the courts all year, and it moved again last month. I broke down what the August ruling covers and who it protects, and Freedom’s Lodge covered the decision itself. The short version: a serialized receiver you buy from a dealer was never in question. If you want the history of how the line got drawn, my serialization explainer walks it through.
Rifle, Pistol, or SBR?
A receiver that has never been assembled into a rifle is legally a virgin receiver, and that gives you options a complete rifle does not have. You can build it into a rifle, a pistol, or (with the right paperwork) a short-barreled rifle.
The SBR path got a lot easier this year. With the tax stamp eliminated on January 1, 2026, the cost math for a short-barreled build changed completely. I tracked what that meant in this year’s NFA reform coverage, and Popular Suppressors tracked the parallel suppressor rulings. Order the receiver first, then decide.
Assembly Order and the Two Torque Numbers
Most first builds go wrong in the same three places, and all three are avoidable with a $20 punch set and a torque wrench.
Work front to back. Install the magazine catch first, because that job needs open access to the magwell and every other part gets in the way once fitted. Take the bolt catch next, and put a strip of tape along the receiver rail before you drive that roll pin. A slipped punch leaves a gouge on the side of the receiver that no finish hides. The trigger guard comes after, and the front ear is the most commonly broken lug on any AR build. Support the ear from underneath with a socket before you press the pin, or you will be shopping for a new receiver instead of a new trigger.
Two torque values are worth writing on the bench. The pistol grip screw wants roughly 35 to 40 inch-pounds. Over-torque it and you strip the receiver boss, which is not repairable. The castle nut on the buffer tube wants 40 foot-pounds and then staking, and the stake is what keeps the tube from backing out under recoil. Skipping the stake is the single most common shortcut in home builds and the reason rifles come apart on the line.
The buffer detent is the last trap. It sits under the buffer tube and launches across the room the moment you relax pressure. Fit the tube far enough forward to capture the detent before you thread it home, and keep a hand over the top.
None of this requires a gunsmith. It requires patience and the right order, and the receiver you chose decides how forgiving the process is. Tight, correctly cut pin holes make every one of these steps easier.
What You Still Need
The receiver is roughly a fifth of the parts count and less than a fifth of the cost. To finish the rifle you need a lower parts kit with the fire control group, a buffer tube assembly with buffer and spring, a stock or brace, and a complete upper. Plan for the upper as the expensive half. Barrel and optic drive the total far more than the receiver does.
If the rifle is headed for the field rather than the range, the barrel choice matters more than anything on this page. Popular Outdoorsman walked through that trade-off in a 6.5 Creedmoor build review that applies to any precision-minded configuration.
Frequently Asked Questions
Do I need an FFL to buy a stripped lower receiver?
Yes. Federal law treats the receiver as the firearm, so it has to ship to a licensed dealer in your state, where you complete a Form 4473 and pass a NICS check. Expect the dealer to charge a transfer fee, commonly $25 to $50.
Is forged or billet better for a first build?
Forged. Same 7075-T6 alloy and process as service rifles, lower price, and it accepts every mil-spec part on the market. Billet buys you a flared magwell and an integrated trigger guard, which are handling refinements rather than strength gains.
Can one receiver become a pistol or an SBR?
A receiver that has never been built into a rifle can become a rifle, a pistol, or a short-barreled rifle. Build it as a rifle first and converting to a pistol raises legal complications, so decide the configuration before the first assembly and check your state law either way.
How much should an AR-15 receiver cost?
Forged receivers from established makers generally run in the low three figures, billet higher, polymer lower. Prices move with demand, so check the maker’s current listing rather than a number in an article. The receiver is rarely where a build goes over on cost.
Will any upper receiver fit my lower?
Mil-spec forged and most billet lowers accept any mil-spec upper. Some billet receivers are sold as matched sets with tighter tolerances and may fit other uppers loosely or not at all, so if the product page calls it part of a set, treat it that way. Every receiver from a licensed maker also carries a permanent serial number, maker name, and location, which is what makes it transferable in the first place.
Final Thoughts
Uppers get swapped. Triggers get upgraded. Optics get sold at a loss and replaced twice. The stripped lower receiver is the one component that stays, carries the serial, and shows up on every transfer document that rifle ever generates. Dimensional figures here come from the mil-spec technical data package as published by makers like Rock River Arms and Aero Precision, the legal definition from 27 C.F.R. § 478.12, and prices are given as ranges because receiver pricing moves with demand (updated September 2, 2026).
Buy the one you would still be happy to see in ten years, because you probably will.
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