AR-15 Parts

Lower Receivers and Lower Parts

The AR-15 lower receiver is the serialized component—the part legally considered the firearm. Stripped lowers are blank canvases for custom builds; complete lowers include the entire fire control group, stock, and pistol grip assembled. Lower parts kits contain every small component needed to complete a stripped lower. For a first-time AR-15 builder, a quality stripped lower from Aero Precision or Anderson paired with a lower parts kit and Magpul stock is the most practical starting point. See: AR-15 Lower Receivers.

Barrels

AR-15 barrel length, profile, twist rate, and caliber all directly affect velocity, accuracy, and application. 16″ is the standard legal minimum; 18″ or 20″ adds velocity for precision use. 1:8 twist is the most versatile for common 5.56 bullet weights. Caliber conversions (.300 Blackout, .224 Valkyrie) require only a barrel and bolt change. See: AR-15 Barrels.

Handguards and Rails

M-LOK and Picatinny handguards provide mounting surfaces for lights, lasers, vertical grips, and other accessories. Free-float handguards improve accuracy by eliminating barrel contact. Carbine-length, mid-length, and rifle-length gas systems determine handguard sizing. See: AR-15 Handguards & Rails.

Triggers

Drop-in trigger groups from Geissele, Timney, and CMC are the single most impactful accuracy upgrade for most AR-15s. Two-stage triggers for precision; single-stage for competition speed. See: AR-15 Triggers.

Stocks

Magpul CTR, STR, and MOE stocks eliminate mil-spec buffer tube wobble and improve cheek weld. Luth-AR MBA-1 adds independent length and height adjustment for precision builds. See: AR-15 Stocks.

Grips

Magpul MOE and MIAD grips improve ergonomics and texture over the standard A2. BCM Gunfighter and similar grips optimize angle for competition use. See: AR-15 Grips.

Muzzle Devices

Flash hiders, muzzle brakes, compensators, linear compensators, and suppressor mounts in standard 1/2–28 threads for 5.56 and 9/16–24 for .308. See: AR-15 Muzzle Devices.

Frequently Asked Questions: AR-15 Parts

What AR-15 parts should I upgrade first?
The trigger is the most impactful upgrade on any AR-15 — a quality drop-in trigger group from Geissele, LaRue, or Rise Armament dramatically improves accuracy and split times for a relatively low cost. After the trigger, a free-float handguard is the second most useful upgrade — it removes pressure on the barrel for improved accuracy and provides attachment points for lights and accessories. Grip and stock upgrades are personal preference; the Magpul MOE/CTR stock and MOE-K2 grip are the most popular value upgrades.

Are AR-15 parts interchangeable between brands?
Most mil-spec AR-15 parts are interchangeable between brands due to standardized dimensions. Lower parts kits, stocks, pistol grips, triggers, and handguards sized for Picatinny or M-LOK rails are broadly compatible. Exceptions include proprietary systems: BCM's GUNFIGHTER charging handles, some non-standard buffer systems, and brand-specific lower receivers with non-mil-spec trigger pockets. When in doubt, specify "mil-spec" when purchasing parts.

Do I need a gunsmith to install AR-15 parts?
Most AR-15 upgrades are DIY-friendly with basic tools. Trigger replacement, grip replacement, stock replacement, and handguard swaps require minimal tools and no gunsmithing. Barrel replacement requires a barrel vise and action wrench. Flash hider/muzzle device installation requires proper timing and a torque wrench. Building a complete lower from a lower parts kit is a common beginner project requiring only a roll pin punch set and a lower block. A qualified gunsmith should install items requiring headspace verification.

What is the difference between mil-spec and commercial buffer tubes?
The buffer tube (receiver extension) determines which stocks fit your rifle. Mil-spec tubes have a slightly smaller diameter (1.148 inches) than commercial tubes (1.170 inches). Most quality AR-15s use mil-spec diameter. Stocks are not interchangeable between the two — always match the stock to your tube diameter. When replacing either, buy a matched set to guarantee compatibility.

See Also: AR-15 Rifles • Triggers • AR-15 Lower Receivers • Handguards & Rails • Stocks & Furniture