51 term
- 5.56 NATO
- The standard AR-15 chambering. Slightly higher pressure than .223 Remington; 5.56 chambers safely fire both, .223 chambers should only fire .223.
61 term
- 6.5 Grendel
- An intermediate cartridge built for extended effective range past 500 yards. Requires a dedicated bolt and magazines.
A1 term
- AR-10
- Large-frame AR platform chambered in .308 Winchester and similar cartridges. Not interchangeable with AR-15 parts.
B5 terms
- Barrel Extension
- The steel piece pinned to the chamber end of the barrel that contains the locking lugs the bolt engages. Interfaces with the upper receiver.
- Barrel Nut
- Threaded nut that clamps the barrel into the upper receiver. Torque spec is typically 30 to 80 ft-lbs until a gas tube slot aligns.
- BCG
- Bolt carrier group. The reciprocating assembly that extracts, ejects, and chambers rounds. Includes bolt, carrier, cam pin, firing pin, and gas key.
- Billet
- Machined from a solid block of aluminum. Allows more aggressive design details than forged receivers but costs more with no meaningful strength advantage.
- Buffer Tube
- Receiver extension that houses the recoil buffer and spring. Mil-spec (1.148") and commercial (1.170") diameters are not interchangeable.
C4 terms
- Carbine Gas
- Gas system length of about 7 inches. Pairs with 10.5" to 14.5" barrels.
- Charging Handle
- Non-reciprocating handle used to manually cycle the action. Mil-spec, extended latch, and ambidextrous designs are the common options.
- Chrome Lining
- A hard chrome coating applied to the bore for corrosion resistance and extended barrel life. Common on mil-spec barrels.
- CMV
- Chrome-moly vanadium steel. Common barrel steel (typically 4150 CMV) that balances durability and heat resistance for standard-profile barrels.
D2 terms
- Direct Impingement
- Gas operating system where propellant gas is routed directly through the gas tube into the bolt carrier. Standard on AR-15s.
- Dwell Time
- The interval a bullet spends in the barrel between the gas port and the muzzle. Too little dwell causes short-stroking; too much causes overgassing.
F3 terms
- Flat-Top
- Upper receiver design with a full-length Picatinny rail on top for direct optics mounting. The modern standard.
- Forged
- Aluminum heated and compressed under pressure into a receiver blank, then machined. The mil-spec method, less expensive than billet.
- Free-Float
- Handguard design that attaches only to the upper receiver and does not contact the barrel. Accuracy standard for modern builds.
G3 terms
- Gas Block
- Component that seals over the barrel's gas port and directs gas into the gas tube. Low-profile blocks fit under free-float rails.
- Gas Port
- Small hole drilled through the barrel wall that bleeds propellant gas into the gas block. Port diameter tunes the system.
- Gas Tube
- Steel tube that carries gas from the block back to the bolt carrier's gas key. Length must match barrel length for reliable cycling.
H2 terms
- Handguard
- Cover surrounding the barrel and gas system that provides the primary accessory mounting surface. Free-float M-LOK is the current standard.
- Headspace
- The dimension between the bolt face and a datum point in the chamber. Must be verified when mixing barrels and BCGs from different makers.
L1 term
- Lower Receiver
- The serialized "firearm" half of the rifle under federal law. Contains the fire control group and requires an FFL transfer.
M5 terms
- M-LOK
- Modular accessory mounting standard developed by Magpul. Uses slots milled directly into the handguard rather than a full Picatinny rail.
- Mid-Length Gas
- Gas system length of about 9 inches. The typical pairing for 16" barrels; softer recoil impulse than carbine-length.
- Mil-Spec
- Conforming to U.S. military specifications for a given part. Ensures interchangeability across manufacturers.
- MOA
- Minute of angle. An angular measurement equal to roughly 1 inch at 100 yards, used to describe accuracy and scope adjustments.
- Muzzle Device
- Flash hider, muzzle brake, or compensator threaded onto the barrel's muzzle. Affects flash signature, recoil, and suppressor compatibility.
N2 terms
- NFA
- National Firearms Act. Federal law regulating short-barreled rifles, suppressors, and other restricted configurations.
- Nitride
- A salt-bath surface treatment (also called Melonite or QPQ) that hardens steel and improves corrosion resistance. Common on barrels and BCGs.
P3 terms
- Phosphate
- Parkerized finish, the mil-spec BCG coating. Durable and matte-black, but requires more lubrication than nickel-boron or DLC coatings.
- Picatinny Rail
- MIL-STD-1913 rail interface. The rail on top of flat-top uppers and older quad-rail handguards uses this standard.
- Pistol Gas
- Shortest common gas system, about 4 inches. Used on 7.5" to 10.5" barrels.
R1 term
- Rifle Gas
- Longest standard gas system, about 12 inches. Pairs with 18" and longer barrels for the softest recoil impulse.
S1 term
- Stripped Upper
- A bare upper receiver with no barrel, BCG, handguard, or gas system. Requires separate parts and an armorer's wrench to build.
T2 terms
- Takedown Pin
- One of two pins (front pivot, rear takedown) that hold the upper and lower receivers together. No tools required to separate.
- Twist Rate
- How quickly the rifling spins the bullet, expressed as 1 turn in X inches (e.g., 1:7). Faster twists stabilize heavier bullets.
U1 term
- Upper Receiver
- The top half of the rifle that houses the barrel, BCG, and charging handle. Determines caliber and barrel length. Not classified as a firearm under federal law.
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