AR-15 Lower Receivers — Stripped, Complete & What to Know
The lower receiver is the only component of an AR-15 that is legally a firearm. It carries the serial number, requires an FFL transfer and background check, and is the foundation to which every other component — trigger group, stock, buffer tube, pistol grip, and magazine well — attaches. Every AR-15 build starts here.
Stripped vs Complete is the primary decision. A stripped lower is the bare serialized receiver with no internal components — the starting point for a custom build where you select and install every trigger, grip, stock, and fire control group yourself. A complete lower comes with all internal parts installed — trigger group, safety selector, pistol grip, buffer tube, and typically a stock — ready to accept an upper receiver and shoot immediately. For first-time builders who want to minimize assembly complexity, a complete lower eliminates the need to source and install lower parts kits. For experienced builders who want to specify a match trigger, specific stock, or enhanced fire controls, a stripped lower is the correct starting point.
Material and construction matter but less than marketing suggests. Forged 7075-T6 aluminum is the mil-spec standard — heated aluminum pressed into shape, producing the densest grain structure and highest strength-to-weight ratio. This is what military M4 carbines use and what the vast majority of quality AR lowers are made from. Billet lowers are machined from a solid block of aluminum, allowing more complex shapes and tighter tolerances but at higher cost and weight. Both will outlast the shooter who builds on them in normal use. A $50 PSA forged lower performs identically to a $300 billet lower from a premium manufacturer in every measurable function test — the premium pays for tighter upper/lower fit, ambidextrous controls, and machining aesthetics.
The Aero Precision M4E1 is the most popular stripped lower in current build data — its threaded bolt catch pin, integrated trigger guard, and optional tensioning screw for tight upper fit make it the practical choice for most builders. PSA's complete lower assembly is the most-added complete lower in build data, driven by its combination of price and ready-to-shoot convenience.
For trigger selection to install in your lower, see our AR-15 trigger guide. All lower receivers transfer through a licensed FFL dealer — they are legally firearms regardless of whether they are stripped or complete.
AR Lower Receiver Frequently Asked Questions
Do I need an FFL transfer for a stripped lower receiver?
Yes. A stripped AR-15 lower receiver is legally classified as a firearm by the ATF regardless of whether it has any components installed. It carries a serial number and requires a standard background check and FFL transfer — the same process as buying any other firearm. The FFL paperwork for a stripped lower typically lists the receiver as "other" or "receiver" under firearm type rather than pistol or rifle, since a bare lower can be built as either. Expect FFL transfer fees of $25–$75 depending on your local dealer.
What is the difference between a stripped and complete lower receiver?
A stripped lower is the bare serialized receiver with no internal components — you install every trigger, safety, magazine release, bolt catch, pistol grip, buffer tube, and stock yourself, either from a lower parts kit or by selecting individual components. A complete lower arrives with all of these components already installed and function-tested, ready to accept an upper receiver. Stripped lowers give you control over every component choice. Complete lowers eliminate assembly complexity. Both are legally firearms and require FFL transfer.
What is the best AR-15 lower receiver for most builders?
The Aero Precision M4E1 is the most recommended stripped lower for most builds — its threaded bolt catch pin eliminates one of the most frustrating steps in AR assembly, the integrated trigger guard is a clean design improvement over mil-spec, and the optional tensioning screw on Gen 2 models creates a tight upper-lower lockup. For complete lowers, PSA's complete assemblies offer the best value for buyers who want to install an upper and shoot without assembly work. For ambidextrous controls, the ADM UIC offers full ambi functionality at a more accessible price than the Radian AX556.
Can I build an AR pistol on any lower receiver?
Only if the lower has never previously been configured as a rifle. A lower that has had a rifle stock installed on it is legally a rifle lower — it cannot be converted to pistol configuration. If you intend to build an AR pistol, start with a lower that was transferred to you as a pistol — one that has never had a rifle stock attached. Since January 1, 2026, the NFA $200 tax stamp has been eliminated for SBRs, making Form 1 registration a practical alternative if you want a traditional stock on a short-barreled configuration. See our SBR guide for the 2026 NFA changes.
Are forged or billet lower receivers better?
Forged lowers are stronger, lighter, and less expensive than billet. Forging compresses aluminum into a dense grain structure aligned to the receiver shape — the same process used for military M4 carbines. Billet lowers are machined from a solid aluminum block, allowing more complex profiles and tighter tolerances, at higher cost and weight. In practical use, both will outlast the shooter — the functional performance difference is negligible. Billet lowers are worth the premium if you want unique aesthetics, tighter upper-lower fit, or specific design features like integrated trigger guards. For most builders, a quality forged lower is the correct choice.