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Firearms 101 - How firearms work, Glossary, Acronyms, Etc.

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Firearms 101

This thread will be used to share the basics of firearms and ammunition, as well provide a glossary of firearms-related terms and acronyms.  Obviously this is a work in progress.  We will be adding more information as the project progresses but we need your help!  Add your definitions, abbreviations, acronyms and suggestions for what you think should go into the Firearms 101 thread. 

The Basics - Firearms
The Basics - Ammunition
Firearms Acronyms and Abbreviations
Firearms Glossary A-L
Firearms Glossary M-Z

Before "class" begins, be sure that you know the FOUR LAWS OF GUN SAFETY!

The Basics - Firearms

Types of Firearms

Common Firearms Types
These will be included in the glossary portion of Firearms 101, but are also included here for convenience and clarity.

Pistol - A short firearm intended to be held and fired with one hand.
Revolver - A handgun having a revolving chambered cylinder for holding a number of cartridges, which may be discharged in succession without reloading.
Rifle - A firearm meant to be held to the shoulder when fired.  Named for the spiral grooves cut in the inner surface of the gun barrel to give impart rotation on the projectile for increased accuracy.
Carbine - A long-gun similar to, but shorter than a rifle
Shotgun - A firearm, usually with a smooth bore capable of firing multiple projectiles (shot, pellets).  May also fire a single projectile (slug) with or without the addition of a rifled barrel.
Machine gun - A small arm operated by a mechanism, able to deliver a rapid and continuous fire of bullets as long as the trigger is pressed.
Submachine gun - An automatic carbine, designed to fire pistol cartridges.
Automatic rifle - A rifle capable of fully-automatic operation; that is, when fired, it will continue to fire until the trigger is released or it is out of ammunition.  Sometimes referred to as an assault rifle.  NOT to be confused with an "assault weapon."
Assault Weapon - A term introduced into usage by gun control groups.  The idea of an assault weapon is notably vague, as any instrument used to assault someone or something is an assault weapon.  In common usage, an assault weapon is identical in operation to legally available semi-automatic or single-shot rifles, pistols or other firearms with the exception of the presence of certain, arbitrary cosmetic features.

Common Types of Firearms Actions
The "action" of a firearm refers to physical mechanism that loads, unloads or otherwise manipulates cartridges and/or seals the breech of the firearm.

Single Shot - A type of action in which the firearm is capable of holding and firing only one round at a time before requiring manual unloading/reloading by the user.  Examples include break-action, tilting-, falling- and rolling-block actions, and some bolt-actions.
Revolver - A revolver houses cartridges in a rotary cylinder and advances them in-line with the bore prior to each shot.
Bolt Action - An action in which the opening and closing of the breech is operated manually by manipulating handle attached to the bolt.
Lever Action - Uses a lever to eject and chamber cartridges.
Pump Action - An action in which the  fore end is manually operated in a sliding or pumping motion by the user to eject and chamber rounds.
Semi-automatic - A type of firearm operation in which the pulling of the trigger causes the chambered round to be fired, the spent case to be ejected and the following round to be chambered for the next shot, which requires a subsequent trigger pull.  Examples include blowback, recoil, direct impingement and gas piston operation.
Automatic - Similar in operation to semi-automatic firearms with the exception that continuing to depress the trigger will cause the firearm to keep firing until out of ammo or the trigger is released.

Firearms - Parts

These diagrams show the basic parts of a variety of firearms

This animation shows the cycling of a semi-automatic pistol.

The Basics - Ammunition

Ammunition - Components
There are a number of types of ammunition.  These types have some common components as well as some unique components:

Centerfire Ammunition Components:
The basic components of modern centerfire ammunition includes the following components:

Case - The outside (usually) metal shell that holds the rest of the components.

Projectile - The bullet portion of the ammunition that is expelled from the muzzle of the barrel.  Projectiles come in many varieties - The one depicted here is a copper-jacketed lead core hollow point projectile.

Powder - The propellant that ignites, creating a volume of hot, expanding gas that propels the projectile down the barrel.

Primer - The primary ignition source that creates a spark to ignite the powder.
Shotgun Ammunition Components:
The basic components of modern centerfire ammunition includes the following components:

Case - The outside shell that holds the rest of the components.

Projectile(s)- The portion of the ammunition that is expelled from the muzzle of the barrel.  Shotshell projectiles come in several varieties - generally shot (birdshot, buckshot) or slugs.

Wad- A cup to hold shot-type projectiles.

Powder - The propellant that ignites, creating a volume of hot, expanding gas that propels the wad andprojectile down the barrel.

Primer - The primary ignition source that creates a spark to ignite the powder.
Rimfire Ammunition Components:
Rimfire ammunition is similar to certerfire ammunition except that, rather than a separate primer cup, the priming material is held in the rim of the cartridge.

Ammunition - How It Works

Most ammunition works the the same general way:  an ignition source ignites a propellant which creates a rapidly expanding volume of gas that forces the projectile down the barrel.

In this animation, you can see the firing pin (light gray) strike the primer (orange), causing an initial spark from the priming material (black).  This spark (orange) ignites the main powder charge (dark gray).  As this powder burns, the reaction creates the rapidly expanding gas (light orange) which forces the projectile (light gray) down the barrel (blue).

Ammunition - Types of Primers

WikiPedia:  BBODO

Ammunition - Common Projectiles
The following are common projectile types and their abbreviations.  These may also be found in the acronyms and abbreviations section

BT - Boat-Tailed
FMJ - Full Metal Jacket
FP - Flat Point
HP - Hollow Point
JHP - Jacketed Hollow Point
JSP - Jacketed Soft Point
LHP - Lead Hollow Point
LRN - Lead, Round Nose
LSWC - Lead Semi-WadCutter
RNL - Round Nosed Lead
SJHP - Semijacketed Hollow Point
SJSP - Semijacketed Soft Point
SP - Soft Point
SWC - Semi-WadCutter
WC - WadCutter

Firearms Acronyms and Abbreviations

2A - Second Amendment

ACP - Automatic Colt Pistol (e.g. .45 ACP)
AE - Action Express (e.g. .50 AE)
AIWB - Appendix Inside Waistband
AK - Avtomat Kalashnikova
ATF - Alcohol Tobacco and Firearms
Anti - short for someone who is anti-Second Amendment
AOW - Any Other Weapon
AP - Armor Piercing
API - Armor Piercing, Incendiary
APT - Armor Piercing, Tracer
AR - Often short for AR15, originally as in Armalite
AWB - Assault Weapon Ban

BATFE - Bureau of Alcohol Tobacco Firearms and Explosives
BAR - Browning Automatic Rifle
BC - ballistic coefficient
BCG - Bolt Carrier Group
BG - Bad Guy
BMG - Browning machine gun
BP - black powder
BT - Boat-Tailed
BUG - Backup Gun - Class 3, see NFA
BUIS - Backup Iron Sights

C&R - Curio & Relic
Cal. - Caliber
CCDW - Carry Concealed Deadly Weapon
CCP - Concealed Carry Permit
CCRKBA - Citizens Committee for the Right to Keep and Bear Arms
CCW - Carry Concealed Weapon
CFP - Concealed Firearms Permit
CHL - Concealed Handgun License
CIII - Class 3, see NFA
CLDF - Civil Liberties Defense Foundation
CMP - Civillian Marksmanship Program
CQB - close quarters battle
CQC - close quarters combat
CSGV - Coalition to Stop Gun Violence, formerly National Coalition to Ban Handguns (NCBH)
CWP - Concealed Carry Permit

DA - Double Action
DAO - Double Action Only
DOD - Department of Defense
DOJ - Department of Justice

EBR - "Evil Black Rifle"
EDC - Every Day Carry

FFL - Federal Firearms License
FMJ - Full Metal Jacket
FN - Fabrique Nationale
FNH - Fabrique Nationale Herstal
FOID - Firearms Owner IDentification
FOP - Fraternal Order of Police
FOPA - Firearms Owner Protection Act
FP - Flat Point
FPS - Feet Per Second, commonly used to measure muzzle velocity
FSB - Front Sight Base
FTE - Failure to Eject (a malfunction)
FTF - Failure to Feed or Failure to Fire (a malfunction)

GA - gauge
GCA - Gun Control Act
GI - Government Issue
GOA - Gun Owners of America
GSSF - Glock Sport Shooting Foundation

HD - Home Defense
H&H - Holland & Holland
HP - Hollow Point
H&R - Harrington & Richardson

IDPA - International Defensive Pistol Association
ILA - Institute for Legislative Action (as in NRA-ILA)
IMR - Improved Military Rifle
IPSC - International Practical Shooting Confederation
IWB - Inside Waistband

JHP - Jacketed Hollow Point
JPFO - Jews for the Protection of Firearm Ownership
JSP - Jacketed Soft Point

KISS - Keep it Simple, Stupid (i.e. something uncomplicated)

LC - Long Colt (e.g. .45 LC)
LE - Law Enforcement
LEO - Law Enforcement Officer
LGS - Local Gun Store
LHP - Lead Hollow Point
LPK - Lower Parts Kit
LR - Long Rifle (e.g. .22 LR)
LRN - Lead, Round Nose
LSWC - Lead Semi-WadCutter

Mag - Magnum or short for magazine
MAIG - Mayors Against Illegal Guns
MG - Machine Gun
mm - millimeter
MOA - Minute of Arc/Angle
MOUT - Military Operations on Urban Terrain

NAGR - National Association for Gun Rights
NATO - North Atlantic Treaty Organization (e.g 7.62 NATO a/k/a 7.62x51)
NBRSA - National Bench Rest Shooters Association
NCBH - National Coalition to Ban Handguns
NEF - New England Firearms
NFA - National Firearms Act
NFRTR - National Firearms Registration and Transfer Record
NICS - National Instant Criminal backgroud check System
NRA - National Rifle Association
NSSF - National Shooting Sports Foundation

OAL - Overall Length
OC - Open Carry or Oleoresin Capsicum (e.g. pepper spray)
O/U - Over/Under
OWB - Outside Waistband

POA - Point of Aim
POI - Point of Impact
PSI - Pounds per square Inch, measurement of pressure

QD - Quick Detach

RDS - Red Dot Sight
Rem - Remington (e.g. .223 Rem)
RKBA - Right to Keep and Bear Arms
RNL - Round Nosed Lead
RRA - Rock River Arms

SA - Single Action or semi-auto
SA/DA - Single Action/Double Action
SAAMI - Sporting Arms and Ammunition Manufacturers Institute
SAA - Single Action Army
SAF - Second Amendment Foundation
SBR - Short Barreled Rifle
SBS - Short Barreled Shotgun
SHOT - Shooting Hunting and Outdoor Trades (e.g. SHOT Show)
SMG - Submachine gun
SJHP - Semijacketed Hollow Point
SJSP - Semijacketed Soft Point
SOB - Small Of Back
SOT - Special Occupational Tax
SP - Soft Point
Spl - Special (e.g. .38 Spl)
SWAT - Special Weapons And Tactics
S&W - Smith & Wesson
SWC - Semi-WadCutter

TC - Thompson Center

USPSA - United States Practical Shooting Association

VFG - Vertical Forward Grip

WC - WadCutter
WCF - Winchester Center Fire
Win - Winchester (e.g. .308 Win)
WML - Weapon Mounted Light
WMR - Winchester Magnum Rimfire (e.g. .22 WMR)
WSM - Winchester Short Magnum (e.g. .300 WSM)

YHM - Yankee Hill Machine

Firearms Glossary  A - L


Action - The physical mechanism that manipulates cartridges and/or seals the breech. 

Accidental Discharge (AD) An unexpected discharge of a firearm due to circumstances beyond the control of the firearm's operator. Often, the AD term is applied to instance of Negligent Discharges (ND). ADs are usually attributed to mechanical failure or parts breakage.

Ammunition (Ammo) - Generally, the assembly of a projectile, gun powder, and primer components contained in a cartridge case.

Assault Rifle A highly regulated firearm which fires an intermediate round and includes select fire capabalities for semi-automatic and automatic fire modes.

Assault Weapon An invented political term with only nebulous definition as proscribed in various federal, state, and local laws. Often used by gun control proponents wishing to regulate such items. Almost universally, this term refers to semi-automatic firearms. Contrast with Assault Rifle.


Ballistics The science of cartridge discharge and the projectile's flight.

Bandolier - Pocketed belt for holding ammunition and cartridges.

Barrel - The tubular portion of a firearm through which a rapid expansion of gases are released to propel a projectile out of the end at high velocity.

Ballistic Coefficient (BC) - The measure of projectile's ability to overcome air resistance in flight.

Beavertail An extended portion of a firearm's grip which protects the user's hand from entering the path of the slide or hammer.

Bedding - A process of filling gaps between the action and the stock of a rifle with an epoxy based material.

Bipod - A two-legged support device which provides a forward rest to reduce movement during firing without having to manually support the firearm.

Birdshot A type of shotgun projectile(s) which uses very small pellets, typically ranging in size from the #9 shot which is approximately .08″ in diameter to #2 shot which is approximately .15″.

Black Powder - A propellant mixture of sulfur, charcoal, and potassium nitrate. Modern firearms typically use smokeless powder instead of black powder, which is generally used in vintage, antique, or muzzle-loading firearms.

Blank - A type of cartridge that contains gunpowder but no projectile. Creates a report and flash as the primer and/or powder ignites. Often used for training, simulation, and signaling.

Blowback - A system of operation for self-loading firearms that uses power from the motion of the cartridge case as it is pushed to the rear by equal-and-opposite forces created by expanding gases during ignition of the propellant.

Bluing (Blued) - A controled oxidization process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish.

Bolt Action - A type of firearm action in which the firearm's bolt is manually operated by opening and closing of the breech with a small handle.

Bore Axis The (theoretical) line which runs through the very center of a firearm's barrel, parallel with the inner walls of said barrel.

Brass - The common term for empty cartridge cases, which are often made of this metal.

Break-Action - A firearm whose barrel(s) is hinged to open and expose the breech to allow the loading and unloading of ammunition.

Breech The rearmost portion of a firearm's barrel closest to the shooter. Typically the portion of the barrel where the cartridge or propellant and projectile are housed for firing.

Breech Loader A firearm in which the ammunition is inserted into a chamber in the rear of a barrel.

Breech Block The part of the action that seals the rear of the chamber while the gun is firing to keep the expansion of burning propellant moving forward through the barrel instead of rearward toward the shooter.

Bullpup - A firearm configuration in which both the action and magazine are located to the rear of the trigger.

Burst Fire - A firing mode enabling the shooter to fire a fixed, predetermined number of rounds with a single pull of the trigger.

Button Rifling - Rifling formed by pulling a die made with reverse image of the rifling (the 'button') down the pre-drilled bore of a firearm barrel.

Buckshot A type of shotgun projectile that uses medium to large pellets of greater than .24″ diameter.

Bullet The projectile expelled from the barrel of a firearm when fired. Not to be confused with a cartridge, which is the complete assembly that includes the projectile, powder, and primer housed in the case.


Caliber - The internal diameter of a firearm's barrel or a cartridge's bullet, usually expressed in millimeters or hundredths of an inch.

Carbine - A shortened version of a rifle.

Cartridge - The assembly consisting of a bullet, propellant, case, and primer.

Centerfire - A cartridge (or firearm using such a cartridge) in which the primer is located in the center of the case head. Contrast with rimfire cartridges and firearms.

Chamber - The portion of the barrel or firing cylinder in which the cartridge is inserted prior to being fired.

Charging Handle - A device on a firearm used to move (cock) the hammer or striker into the ready position.

Choke - A tapered constriction of a shotgun barrel at the muzzle end to control the pattern of the expelled projectiles in flight.

Clip - A device that is used to store multiple rounds of ammunition together for insertion into the magazine of a repeating firearm.

Combination Gun - A firearm with multiple barrels often of different calibers, or even different types (such as one rifle and one shotgun barrel).

Co-Witness The configuration of traditional sights plus optical sights such that both sights align and can be used either simultaneously or independently in the case that one fails.

Cross-Dominant (eyes) - A scenario in which a shooter is right-handed but left-eye dominant or vice versa.

Cylinder A rotating drum of cartridge-holding chambers in a revolver.


Direct Impingement - A type of gas operation for a firearm that directs gas from a fired cartridge directly to action to manipulate the action to eject the spent cartridge and load the next.

Double Action/Single Action (DA/SA) A type of action that is designed to operate in double action (with the triger both cocking and releasing the hammer or striker) on the first shot and in single action (with the trigger only acting to release the hammer or striker) on subsequent shots.

Double Action Only (DAO) A type of action in which the trigger both cocks and releases the hammer or firing pin on every shot.

Double Tap Two shots fired in rapid succession.

Drum Magazine - A cylindrical, typically higher capacity, firearm magazine.

Dry Fire - The practice of manipulating the trigger of an unloaded firearm to allow the hammer or striker to fall on an empty chamber.

Dummy - An inert round of ammunition that contains no primer, propellant, or explosive charge.


Eye Relief - The distance from an optic's eyepiece to the viewer's eye to allow proper viewing.

Extractor - A part in a firearm that serves to remove the cartridge case from the chamber.


Failure to Extract (FTE) A malfunction of a semi-automatic firearm in which the extractor fails to remove the empty case from the chamber after firing the round.

Failure to Feed (FTF) A malfunction of a semi-automatic firearm in which a new round is not loaded, or loaded incorrectly or incompletely, into the chamber as the action returns to battery.

Failure to Fire (FTF) A malfunction which results in no shot being fired upon pulling the trigger. Can be a failure on the part of the firearm or the ammunition.

Ferritic Nitrocarburizing - A case hardening processes that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures to improve scuffing resistance, fatigue properties and corrosion resistance of metal surfaces. Also called nitriding as well as number of trade terms used by various manufacturers (Black Nitride, Tennifer, Tenifer, Melonite).

Firearm A rifle, shotgun or handgun which uses a propellant to expel a projectile or projectiles from the barrel.

Forcing Cone - The tapered section at the rear of the barrel of a revolver that eases the entry of the bullet into the bore.

Forward Assist - A device to help manually force the action of a firearm into battery.

Fouling - The accumulation of burned or unburned powder, carbon, lead, copper, or lubrication residue, or bullet material such as lead or copper on surfaces of a firearm such as the bore or action.

Fouling Shot - A shot fired through a cold/clean bore intended to leave firing residue to allow more consistent performance in subsequent shots.

Frangible - A bullet that is designed to disintegrate into particles upon impact to minimize penetration.

Gauge - A unit of measurement used to express the diameter of the barrel, expressed as the multiplicative inverse of the sphere of a given gauge's weight as a fraction of a pound.  E.g. a one-twelfth pound ball fits a 12-gauge bore; thus there are twelve 12-gauge balls per pound.


Grain - Unit of measurement of mass. Used to denote the amount of powder in a cartridge or the weight of a projectile. Per the International System of Units, 64.79891 mg.

Grip Safety - A safety mechanism integrated into the rear of a pistol grip which unlocks the trigger mechanism of a firearm as the grip is held by the shooter.


Hang Fire - A delay (typically undesired and unexpected) between the manipulation of the trigger of a firearm and the ignition of the propellant. SAFETY NOTE - If you experience a hang fire, keep your firarm pointed in a safe direction for a suitable period of time (one or more minutes) before investigating the malfunction to ensure that the firearm will not fire unexpectedly.

Half-Cock - The position of a hammer where the hammer is partially but not completely cocked, which often acts as a type of safety and/or facilitates loading.

Hammer - The part of a firearm's action that either strikes a cartridge's primer directly, or strikes a firing pin which, in turn, strikes the primer to cause ignition.

Headspace - The distance measured from the part of the chamber that stops forward motion of the cartridge to the face of the bolt.

Headstamp - The markings on the rear of a cartridge case which typiclaly notes the cartridge's caliber, manufacturer, or other information.

High Brass - A shotgun shell for more powerful loads, visually distinguishable by the brass extending up further along the sides of the shell.

Hollow Point A bullet with a concave nose to increase expansion upon penetration of a solid target.

Holographic Sight - A non-magnifying weapon sight that allows the user to look through a glass optical window and see a reticle image superimposed at a distance on the field of view.

Hoplophobe - A person with an irrational fear of weapons. Coined by retired Marine colonel Jeff Cooper in 1962 from the Greek ὅπλον hoplon, meaning, amongst other things, "arms," and φόβος phobos, meaning "fear."


Iron Sights - A system of aligned markers used to assist in the aiming. Typically comprised of a front and rear sight which are aligned to aim.


Jacket - A metal, usually copper, wrapped around a lead core to form a bullet.

Jeweling - A cosmetic process to enhance the looks of firearm parts, such as the bolt.


Keyholing - The failure of a firearm to properly stabilize the projectile being fired, reulting in the projectile tumpling through the air rather than spinning on its axis. Named for the shape created by the projectile striking the target with its edge rather than its tip.

Kick - The perceived recoil ("felt recoil") imparted rearward on the shooter or firearm mount created by the equal-and-opposite momentum caused by propelling the projectile forward.


Length of Pull (LOP) - The distance between the trigger and the butt end of the stock of a rifle or shotgun.

Lever Action - A type of firearm action in which the shooter manipulates a lever, typically integrated into the trigger guard area, to eject a spent case and load fresh cartridges into the chamber.


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