AR-15 Terminology: A Builder's Reference

What This Article Covers

This guide defines the terms and acronyms that appear across AR-15 guides, forums, and product listings. It is organized by category rather than alphabetically so that related terms can be read together. Use it as a reference alongside the more focused guides on individual components.

Platform Terms

AR-15: The ArmaLite Rifle Model 15, designed by Eugene Stoner in 1959. The civilian semi-automatic variant of the military M16/M4 family. The term is commonly used to describe any rifle built on the AR-15 lower receiver platform.

AR-10: The larger-frame predecessor to the AR-15, chambered in .308 Win / 7.62 NATO and similar cartridges. Shares the same operating principle but uses larger, non-interchangeable components.

AR-9: An AR-15-pattern lower and upper configured for pistol calibers, most commonly 9mm. Uses a dedicated bolt carrier group and barrel; feeds from Glock, Colt, or proprietary magazines depending on the lower.

M16: The military designation for the selective-fire AR-15 adopted by the U.S. Army in 1964. The M16A1, A2, A3, and A4 are successive variants. Colloquially used to distinguish military automatic-capable rifles from semi-automatic civilian AR-15s.

M4: A shorter, carbine-length variant of the M16 with a 14.5-inch barrel and collapsible stock. The M4A1 is the current standard U.S. infantry carbine.

Mil-spec: Short for military specification. Components described as mil-spec meet dimensional and material standards defined by military contracts. A useful baseline for interoperability and known reliability, but not a guarantee of quality on its own.

SOPMOD: Special Operations Peculiar Modification. Refers to the accessory kit and variants of the M4 used by U.S. special operations forces. Often used loosely to describe modular, accessory-laden builds.

Lower Receiver Terms

Lower receiver: The serialized, regulated portion of the AR-15 that houses the trigger group, magazine well, and buffer tube. The lower is the legal firearm — all other components are unregulated parts.

Stripped lower: A bare lower receiver with no internal parts installed. Requires a lower parts kit and stock/buffer assembly to function.

Billet lower: A lower machined from a solid block of aluminum. Typically tighter tolerances and more cosmetic options than forged; not inherently more reliable.

Forged lower: A lower formed by pressing aluminum into a die under high pressure, then machined to final dimensions. The most common manufacturing method; consistent and proven.

LPK (Lower Parts Kit): The set of small parts that complete a stripped lower — trigger group, hammer, disconnector, safety selector, bolt catch, magazine release, trigger guard, grip, and associated pins and springs.

FCG (Fire Control Group): The trigger, hammer, and disconnector as a functional set. Replacement FCGs (drop-in triggers) replace these three parts as a unit.

MIL-SPEC buffer tube: Also called a carbine receiver extension. A buffer tube that meets military dimensional specifications — six-position with a 1.148-inch outer diameter. Stocks labeled “mil-spec” fit this tube. Commercial buffer tubes use a 1.168-inch OD and require commercial-spec stocks.

Upper Receiver Terms

Upper receiver: The non-serialized upper half of the AR-15 that houses the barrel, bolt carrier group, and charging handle. Complete uppers can be swapped between mil-spec lowers without tools.

Complete upper: An upper receiver with the barrel, handguard, gas system, and sometimes the bolt carrier group and charging handle already installed.

Stripped upper: A bare upper receiver with no components installed.

Dust cover: The spring-loaded cover over the ejection port that keeps debris out of the action when closed. Pops open automatically on the first shot.

Forward assist: A serrated button on the right side of most upper receivers used to manually push the bolt into battery. Absent on some lightweight and competition uppers.

A2 upper: An upper receiver with a fixed carry handle and integrated rear sight. Named for the M16A2 configuration. Less common on modern builds but still used.

Flat-top upper: A Picatinny rail (MIL-STD-1913) across the top of the upper receiver with no fixed carry handle. The standard for modern builds; allows optic mounting at any position.

Barrel and Gas System Terms

Barrel nut: The threaded collar that secures the barrel to the upper receiver. Also the attachment point for most handguards.

Gas port: The small hole drilled in the barrel that diverts propellant gas to operate the action. Located at different positions along the barrel depending on gas system length.

Gas system length: The distance from the upper receiver to the gas port. Common lengths: pistol (shorter than carbine), carbine (~7”), mid-length (~9”), rifle (~12”). Longer gas systems reduce the pressure and velocity at which gas enters the key — generally producing softer cycling.

Gas key: The rectangular fitting on top of the bolt carrier group that accepts gas from the gas tube and drives the bolt carrier rearward.

Dwell time: The time between bullet exit from the gas port and exit from the muzzle. Longer dwell time allows more gas to enter the system. Suppressed rifles increase dwell time, contributing to overgassing.

Carbine-length gas system: Gas port at approximately 7 inches from the receiver. Common on shorter barrels (10”–16”). Runs at higher pressure than mid or rifle length.

Mid-length gas system: Gas port at approximately 9 inches. A popular choice for 14.5”–16” barrels; balances reliability with softer cycling compared to carbine length.

Rifle-length gas system: Gas port at approximately 12 inches. Used on 18”–20” barrels. The original AR-15 spec; softer recoil impulse.

Adjustable gas block: A gas block with a metered port that allows the builder to restrict or increase gas flow. Useful for suppressed builds, precision setups, or rifles running a range of ammunition.

Low-profile gas block: A gas block with a smaller footprint than the original front sight block, required when using a free-float handguard. Available in set-screw, clamp, or pinned styles.

Bolt Carrier Group Terms

BCG (Bolt Carrier Group): The reciprocating assembly that chambers rounds, fires them, and ejects spent cases. Includes the carrier, bolt, firing pin, cam pin, and extractor.

Full-auto BCG: A BCG with an extended lower tail on the carrier. Legal in semi-automatic rifles; the tail geometry is a manufacturing convenience, not a function difference in semi-auto rifles.

Cam pin: The pin that rotates the bolt to lock and unlock it from the barrel extension during cycling.

Extractor: The spring-loaded claw on the bolt that grips the cartridge rim to pull spent cases from the chamber on extraction.

Ejector: The spring-loaded plunger in the bolt face that kicks the spent case out of the ejection port after extraction.

Headspace: The distance between the bolt face and the chamber datum line. Correct headspace is required for safe, reliable function. Verified with go/no-go gauges when assembling a barrel and bolt.

Handguard and Rail Terms

Free-float handguard: A handguard that attaches only to the upper receiver — not the barrel or gas block. Eliminates pressure on the barrel from hand grip, which improves accuracy consistency.

Drop-in handguard: A handguard that uses the factory delta ring and cap to retain against both the upper receiver and barrel nut. Easier to install; applies slight pressure to the barrel.

M-LOK: A slot-based accessory attachment system developed by Magpul. Widely adopted on modern handguards; allows mounting of lights, grips, slings, and other accessories.

Picatinny rail (MIL-STD-1913): A standardized rail profile with evenly spaced cross-slots. Common on older and mil-spec handguards; heavier than M-LOK but compatible with the widest range of accessories.

KeyMod: An earlier slot-based system from VLTOR/Noveske. Functionally similar to M-LOK but less commonly specified on new handguards.

Muzzle Device Terms

Flash hider: A muzzle device that disperses burning propellant gases to reduce visible muzzle flash. The standard military muzzle device. Does not significantly affect recoil.

Compensator: A muzzle device that redirects gas upward or laterally to counter muzzle rise and movement. Improves follow-up shot speed.

Muzzle brake: A muzzle device that vents gas laterally or rearward to reduce recoil force. Most effective recoil reduction; increases concussion to the sides.

Direct thread suppressor: A suppressor that threads directly onto the muzzle. Requires a matching thread pitch (most common: 1/2x28 for .223/5.56).

QD suppressor mount: A quick-detach mounting system that allows suppressor removal without tools. Common systems include Surefire SOCOM, AAC 51T, and Dead Air Keymo.

Suppressor host: A rifle configured and optimized for running a suppressor. Typically includes a compatible muzzle device and tuned gas system.

Stock and Buffer Terms

Collapsible stock: A stock that adjusts rearward on a carbine buffer tube to multiple length-of-pull positions. The standard on most carbine-length builds.

Fixed stock: A non-adjustable stock. Common on rifle-length builds with A2 or similar fixed configurations; A2 buttstock is the mil-spec fixed stock.

Pistol brace: A device designed to stabilize a pistol-configuration AR against the forearm. Does not legally constitute a stock. See the Stocks and Braces guide for current legal context.

Castle nut: The large notched nut that locks the buffer tube to the lower receiver. Staked or Loctited to prevent backing off under recoil.

End plate: The plate that sits between the castle nut and lower receiver. Some end plates include sling mount points.

Ammunition Terms

5.56 NATO: The NATO-standard cartridge for the AR-15. Loaded to higher pressure than .223 Rem. Safe to fire in 5.56 and .223 Wylde chambers.

.223 Remington: The SAAMI-standardized commercial cartridge. Dimensionally similar to 5.56 but lower pressure. Safe in all three chamber types; not recommended in a .223 Rem chamber with 5.56 loads. See 5.56 vs. .223 Wylde vs. .223 Rem.

BTHP: Boat-tail hollow point. A match-grade bullet profile with a tapered base and hollow tip for consistent expansion and accuracy. Common in precision ammunition.

FMJ: Full metal jacket. A bullet with a lead core enclosed in a harder metal shell. Standard for practice and range use.

NATO headstamp: A cross-in-circle symbol on the cartridge base indicating the round meets NATO pressure specifications. Confirms it is 5.56 NATO, not .223 Rem.

Grain (gr): The unit of measurement for bullet weight. Heavier bullets (62gr, 77gr) are slower but retain energy at distance; lighter bullets (55gr) move faster with flatter trajectory at shorter ranges.

Legal and Regulatory Terms

Title I firearm: A standard firearm regulated under the Gun Control Act — requires an FFL for transfer but no special tax or registration. Most AR-15 rifles are Title I.

NFA item: A firearm or device regulated under the National Firearms Act. Requires a $200 tax stamp and ATF approval. See NFA Basics for AR-15 Builders.

FFL (Federal Firearms Licensee): A federally licensed dealer authorized to transfer firearms. Required for most commercial firearm transfers. See Buying Firearms Online: The FFL Transfer Process.

SBR: Short-Barreled Rifle. An NFA-regulated rifle with a barrel under 16 inches or overall length under 26 inches.

Form 4: The ATF form used to transfer an existing NFA item from one party to another. Requires ATF approval before transfer.

Form 1: The ATF form used to manufacture an NFA item. Used when a builder wants to make their own suppressor or SBR.

Tax stamp: The approval document issued by the ATF after processing a Form 1 or Form 4. The $200 fee has not changed since the NFA was enacted in 1934.