Gun & Firearm Glossary: 30 Key Terms Explained

Quick answer: This glossary defines the 30 firearm, ammunition, and optics terms that come up most in reviews and buying guides — from striker-fired and MOA to NFA and twist rate — in plain English. Jump to a section below.

Actions & Fire Control

Action

The mechanism that loads, fires, and ejects a cartridge. Common types include bolt, lever, pump, and semi-automatic.

Striker-fired

A pistol with no external hammer; a spring-loaded striker is released by the trigger to fire the round (e.g., Glock, most modern polymer pistols).

Hammer-fired

A design in which an external or internal hammer strikes the firing pin. Often paired with DA/SA triggers.

DA/SA (double/single action)

The first shot is a long, heavy double-action pull that both cocks and releases the hammer; follow-up shots are a short, light single-action pull.

DAO (double-action only)

Every trigger pull is a full double-action stroke, giving a consistent (but longer) pull shot to shot.

Semi-automatic

One round fires per trigger pull, and the action automatically loads the next cartridge from the magazine.

Trigger reset

The short travel the trigger must return through after a shot before it will fire again — a shorter reset speeds up follow-up shots.

Ammunition & Ballistics

Caliber

The diameter of the bullet or bore, given in inches (.223, .45) or millimeters (9mm, 10mm).

Grain (gr)

A unit of weight (1/7,000 of a pound) used for bullets and powder charges; heavier bullets (higher grain) generally hit harder but drop faster.

FMJ vs JHP

Full Metal Jacket rounds are cheap and used for practice; Jacketed Hollow Points expand on impact and are used for self-defense.

Muzzle velocity

The speed of the bullet as it leaves the barrel, measured in feet per second (fps).

Twist rate

How fast the rifling spins the bullet, written as a ratio like 1:7 (one full turn every 7 inches). Faster twists stabilize longer, heavier bullets.

+P

Ammunition loaded to a higher-than-standard pressure for more velocity; only use it in firearms rated for it.

Subsonic

Ammunition that stays below the speed of sound (about 1,100 fps), avoiding the supersonic “crack” — ideal for suppressed shooting.

Optics & Sights

MOA (minute of angle)

An angular measurement equal to about 1 inch at 100 yards. Used for both accuracy (“a 1-MOA rifle”) and scope adjustments.

MRAD / mil

A metric angular unit equal to about 3.6 inches at 100 yards; the basis for mil-dot reticles and turrets.

Red dot / reflex sight

A non-magnified optic that projects an illuminated aiming dot, allowing fast target acquisition with both eyes open.

Footprint

The bolt-pattern and shape a red dot uses to mount to a slide or plate (e.g., RMR, Docter, RMSc). Matching footprints is what makes an optic fit a given pistol.

Parallax

The apparent shift of the reticle against the target when you move your eye off-center; higher-end optics minimize it.

Zero

The distance at which a sight is calibrated so the point of aim matches the point of impact (e.g., a “25-yard zero”).

Co-witness

Setting up iron sights and a red dot so both line up on target, giving a backup aiming reference.

Parts & Furniture

Cerakote

A durable ceramic-based coating applied to firearms for corrosion resistance and color/finish.

Picatinny rail

A standardized slotted mounting rail (MIL-STD-1913) for optics, lights, and accessories.

M-LOK

A free-float handguard attachment standard (by Magpul) that lets you bolt accessories directly to slots without a full rail.

Chassis

A rigid metal frame — common on precision rifles and some PDW-style pistols — that the barreled action drops into for consistency and modularity.

Threaded barrel

A barrel with muzzle threads for mounting a suppressor, muzzle brake, or flash hider.

Legal & Class

NFA

The National Firearms Act of 1934 — the federal law that regulates suppressors, short-barreled rifles/shotguns, machine guns, and other “Title II” items, typically requiring a tax stamp and registration.

SBR (short-barreled rifle)

A rifle with a barrel under 16 inches (or overall length under 26 inches); NFA-regulated.

FFL (Federal Firearms License)

The license a dealer holds to sell firearms; online gun purchases ship to a local FFL for the background check and transfer.

Constitutional (permitless) carry

State laws that allow carrying a concealed firearm without a permit for those legally allowed to own one.

Definitions are general educational references and not legal advice; firearm laws vary by state and change — verify current NFA and carry rules for your jurisdiction before acting.