1. Additional barrels that may be installed on a frame or receiver for the purpose of
altering the caliber, gauge or barrel length of a firearm. An accessory barrel may have
an attached fore-end or other attachments.
2. Any barrel that changes the caliber of a firearm, that is truly a barrel only and does
not have a breech mechanism of a kind or type normally associated with the parent
firearm, is deemed to be an Accessory Barrel, whether or not it is attached to a barrel
extension or “upper receiver.”
AIR, SPRING OR GAS FIREARM
A firearm design that uses compressed air, compressed gas or a spring to propel a projectile.
One or more loaded cartridges consisting of a primed cartridge case and propellant. May or may
not contain one or more projectiles (Examples: Ball Ammunition, Blank Ammunition and Duplex
A generic term for rim-fire and center-fire ammunition having a metallic cartridge case.
Ammunition that has the propellant charge molded to the base of the bullet or molded around the
bullet, not enclosed in any kind of a cartridge case.
AUXILIARY CHAMBER ADAPTERS
A device that may be inserted into a firearm to adapt the original chamber to fire a different
cartridge. This adapter may have a section of barrel attached.
The study of a projectile in motion.
The study of the motion of a projectile after it leaves the barrel of the firearm.
The study of the motion of projectile(s) within the bore of a firearm from the moment of ignition
until exit from the barrel.
The study of the projectile’s impact effect on the target.
A barrel that has been fired while containing a bore obstruction, without catastrophic failure. The
resultant excessive radial pressure causes circumferential bulge in the barrel.
The elapsed time from the ignition of a cartridge and the bullet’s leaving the cartridge case neck
to the time the bullet exits the barrel.
A shotgun or rifle without the stock.
That portion of a firearm through which a projectile or shot charge travels under the impetus of
powder gasses, compressed air or other like means. A barrel may be either smooth or rifled.
This system in its simplest form relies on the mass of the breech block and the strength of the
return spring to accomplish the locking cycle in keeping with the law of physics dealing with
inertia. The recoiling mass being much greater than the mass of the projectile, there is a time
delay in the opening of the breech until the residual gas pressures have dropped to a sate
A magazine-repeating firearm design in which the breech bolt or closure device operates in line
with the bore. This type of action is manually operated by a permanently fixed projection or
handle attached to the bolt or closure device, usually known as the bolt handle.
The locking and cartridge head support mechanism of a firearm that operates in line with the axis
of the bore (Examples: Colt AR 15 rifle, Winchester Model 70).
A device that allows the bolt to be removed from the firearm.
A device that retains the bolt in the firearm during normal operation.
The locking and cartridge head support mechanism of a firearm that does not operate in line with
the axis of the bore (Example: FN FAL rifle, Bren gun).
Any firearm loaded with a cartridge or propellant from the rear of the bore.
BREECH LOADING, SINGLE BARREL-SINGLE SHOT OR MULTIPLE BARREL-MULTIPLE SHOT FIREARMS
Firearms in which the action of loading is accomplished one cartridge at a time in each barrel,
usually by hand. The breech is then manually closed, a shot or shots are fired, the breech is
opened manually, and the loading action is repeated by hand for each barrel to be discharged.
In percussion and flint-lock firearms, the metal part that is threaded into the breech end of the
barrel and usually incorporates the top tang.
That portion of the receiver or frame of a revolver or break-open single-shot or multi-barrel firearm
that supports the head of the cartridge when it is fired.
An obsolete term referring to a bullet with an exposed lead tip for the purpose of expansion upon
entering a medium with greater density than air. Manufactured at the British Arsenal located at
A bullet containing a chemical compound that ignites on impact to start fires.
A bullet containing a primer or other explosive designed to explode on impact.
1. Firearms--the approximate diameter of the circle formed by the tops of the lands of a
2. Ammunition--a numerical term, without the decimal point, included in a cartridge name
to indicate the approximate bullet diameter.
CAPE RIFLE/CAPE GUN
A double-barreled shoulder arm with barrels side by side, one being smooth bore and the other
A small, generally cylindrical metallic cup containing a primary explosive used to ignite the
powder charge in muzzle-loading (and some breech-loading) firearms.
A rifle of short length and light weight originally designed for mounted troops.
The container for all other components that comprise a cartridge.
An inert cartridge that cannot be fired.
An interior constriction at or near the muzzle end of a shotgun barrel bore for the purpose of
controlling shot dispersion.
CLIP/ CHARGER CLIP/ STRIPPER CLIP
A separate cartridge container/device to hold cartridges for the purpose of rapid reloading. In the
case of certain firearms the charger clip becomes an integral part at the firearm mechanism
(Example: U.S. Rifle M-1 Garand). The term “clip” is also used erroneously to describe cartridge
A stage in the cycle of operation of a firearm in which the firing mechanism is placed under spring
A firearm having more than one barrel installed on the receiver in which the barrels are
permanently attached to each other. In the case where there are two barrels, each barrel must be
chambered for a different caliber cartridge. In the case of a Combination Gun having more than
two barrels, the individual barrels of the firearm will be chambered for at least two different caliber
cartridges. This firearm may be found with two, three or even four barrels in any combination of
rifle or shotgun gauges or calibers.
A firearm design that is a semiautomatic version of a traditional military and police full-automatic
firearm. Commercial version type code CV describes a firearm that, as it left the manufacturer,
was designed and intended to fire semiautomatic only. The type code CV describes only those
firearms that would otherwise be unclassifiable by TYPE according to the existing system when
the full-automatic capability is removed during manufacture.
A device attached to or integral with the muzzle end of the barrel to utilize propelling gasses for
counter recoil. Also called a muzzle brake.
Any device that changes the caliber of the parent firearm, consisting of an assembly of parts; as a
minimum comprised of a barrel and a breech mechanism of a kind or type normally associated
with the parent firearm and having a breech face capable of supporting the cartridge head.
A firearm that, as originally designed and manufactured, had the capacity of firing projectiles in
rapid succession during one pressure of the trigger and has subsequently been altered to fire
only one projectile with one such pressure.
The ignition of a cartridge by overheating.
A firearm manufactured in visual imitation of another firearm with or without the benefit of
permission or license. These copies are normally manufactured to gain an advantage in sales,
marketing, or research and development costs at the expense of the company whose firearm is
being copied. Copies are not normally dimensionally interchangeable with the firearm being
copied. Also known as a counterfeit copy or knock-off.
The term convertible is applied to revolvers in which the cylinder may be interchanged with other
cylinders to permit the firearm to discharge cartridges of different calibers with the same
approximate bullet diameter. When this concept is applied to firearms other than revolvers, for
correct terminology refer to ”Auxiliary Chamber Adapters,” “Accessory Barrels” and “Conversion
An obsolete barrel-making process. The barrel is formed by twisting or braiding together steel and
iron wires or bars. The resulting cable is then wound around a mandrel and forged into a barrel
tube. Sometimes called a “laminated barrel.”
To burn with great heat and light, producing large volumes of gas. Smokeless gun powder is said
A generic term applied to a large variety of pocket-size pistols of any ignition system, single or
multiple barreled. Sometimes spelled Deringer.
Exothermic reaction of a high explosive; an extremely violent reaction at supersonic speed.
A handgun mechanism in which a single pull of the trigger cocks and releases the hammer or
A dummy cartridge supplied by the military for troops to practice loading and unloading drills with
firearms. See also Cartridge, Dummy.
A three-barreled shoulder arm in which a combination of smooth-bored and rifled barrels are
Firing of an unloaded firearm to practice firearm handling and shooting skills.
A stage in the cycle of operation of a firearm in which a cartridge or fired cartridge case is
expelled from the breech of a firearm.
A portion of the firearm mechanism that ejects or expels cartridges or fired cartridge cases when
the action is opened.
A stage in the cycle of operation of a firearm in which a cartridge or fired cartridge case is
withdrawn from the chamber of a firearm.
A mechanism for withdrawing the cartridge or fired cartridge case from the chamber of a firearm.
A stage in the cycle of operation of a firearm in which a fresh cartridge is introduced into the
mechanism from an ammunition supply in such a way that the breech mechanism may insert the
cartridge into the chamber. Feeding may also be accomplished manually.
A weapon from which a dangerous projectile may be propelled by an explosive, or by gas or air.
Firearm does not include a smooth bore rifle or handgun designed and manufactured exclusively
for propelling by a spring, or by gas or air, BB’s not exceeding .177 caliber.
A stage in the cycle of operation wherein the firing mechanism is released by the sear and the
primer of the cartridge is struck by the firing pin to fire a shot.
That part of a firing mechanism that strikes the primer of a cartridge to initiate ignition of the
A thin, sub-caliber, fin-stabilized small dart or arrow encased in a discarding sabot and loaded
into small arms ammunition or a shotgun shell.
1. Lock Mechanism--firing mechanism of a muzzle-loaded firearm wherein the frizzen is
integral with a hinged pan cover. The strike of the flint on the frizzen produces sparks and
simultaneously pivots the pan cover to expose the fine-grain black powder priming charge
contained in the pan.
2. Firearm--muzzle-loading firearm utilizing a flint-lock ignition mechanism. There have
been breech-loading, flint-lock firearms (Example: Ferguson rifle).
In flint-lock mechanism, the steel part placed over the pan against which the flint strikes,
producing the sparks necessary to ignite the black powder priming charge.
A firearm with the capacity to fire projectiles in rapid succession during one press of the trigger.
Method of operation of semiautomatic and automatic firearms in which the force derived from the
propellant gases generated upon firing a cartridge is utilized to complete the cycle of operations.
Gas operated firearms usually depend on springs to store energy for the purpose of feeding,
loading, cocking and locking the mechanism.
A mechanical safety in the grip of some firearms that prevents firing until it is depressed by the
firing hand; this engages automatically when released to prevent the firearm from discharging.
Any firearm designed, altered or intended to be aimed and fired by the action of one hand.
An ammunition malfunction in which the cartridge ignition takes place after an appreciable delay
(fraction of a second to several seconds) after the primer has been struck.
The distance from the closed breech face of the firearm mechanism to the surface in the chamber
upon which the cartridge case bores.
A mechanical device for measuring the distance from the breech face of a firearm mechanism to
that portion of the chamber against which a cartridge will bear. Headspace gauges are available
in three types: minimum, maximum, and field gauges.
HEADSPACE GAUGE – PURPOSE AND USE
The purpose of measuring headspace is to insure that the firearm mechanism will lock correctly
when a maximum tolerance cartridge is loaded into a minimum tolerance firearm chamber
(Minimum Headspace) and that the firearm is safe to fire a minimum tolerance cartridge in a
maximum tolerance firearm chamber (Maximum Headspace) without the possibility of damage to
the firearm or injury to the shooter or close spectators. Headspace gauges are used to decide
whether to permit a firearm with headspace in excess of the maximum tolerance to continue to be
used in military service for emergency requirements, and those with headspace in excess of the
field requirement considered too dangerous for emergency use. Field headspace gauges have no
legitimate civilian or commercial application.
Numbers, letters and symbols stamped on the head of a cartridge case during manufacture to
identify the manufacturer, caliber or gauge, date of manufacture or other information.
The name of an individual or company that brings goods from off shore to the United States of
America or Canada for resale or distribution.
That area forward of the chamber of a firearm that is groove diameter or slightly larger, extending
forward to the origin or commencement of rifling. Also known as the Leed, Lead, Throat,
Chamber Throat or Forcing Cone.
A magazine-repeating firearm design wherein the breech mechanism is cycled by an external
lever, generally below the receiver or frame.
A stage in the cycle of operation of a firearm in which the cartridge is fully inserted into the
chamber in preparation for the locking cycle.
A stage in the cycle of operation of a firearm in which the bolt, breech block or standing breech is
solidly secured in a fixed relationship with the chamber, so as to resist being blown open by the
pressures generated by firing.
The manufacturer is the individual or company that manufactures or subcontracts the
manufacture of any or all component parts of a firearm. It may be possible to identify the fully
assembled firearm to the manufacturer if the firearm has been marked with the manufacturer’s
company name or trademark.
Where the date of manufacture is found marked on a firearm of government contract or
government arsenal production, the date of manufacture shall be included with the model, when
necessary, to uniquely identify that particular firearm from all others of its type.
The make is normally the company name, abbreviation of the company name, brand or trade name
associated with the particular firearm.
1. A container for cartridges that has a spring and a follower to feed those cartridges into
the chamber of a firearm. The magazine may be detachable or an integral part of the firearm.
2. A storage place for gun powder, ammunition or explosives.
A firearm, as originally designed and manufactured, with the capacity of firing projectiles in rapid
succession during one press of the trigger, whether or not it has been altered to fire only one projectile
with one such press.
A firearm fed by an internal or external magazine.
A mechanical safety mechanism found on some semiautomatic pistols that prevents firing when
the magazine is removed from the firearm.
An ammunition malfunction in which firearm fails to discharge after the firing pin has struck an
adequate blow to the primer and the initiated primer fails to ignite the propellant powder.
A multiple-barrel firearm designed with the barrels permanently attached to each other and
capable of discharging multiple shots, contingent on the number of barrels.
The end of the barrel from which the projectile emerges.
Any of the various forms of muzzle treatment designed/meant primarily to protect the rifling.
It may take the form of a counter-bore, radius or chamfering of the muzzle.
The resultant over-pressure blast /noise that occurs at the muzzle of a firearm when the projectile
exits the barrel and the rapidly expanding propellant gasses are released.
Any firearm that is loaded with gun powder and projectile(s) through the muzzle end of the bore.
OPERATION CYCLE, FIREARM
A series of mechanical or manually operated events that take place in all firearms during the
discharge of ammunition. These terms are used to describe with technical accuracy the manner in
which the mechanism functions. This series of events is described as follows:
(NOTE: These events do not necessarily occur in the order described above.)
The word “original” refers to a model of firearm as produced by the manufacturer who first introduced
that model for commercial or contract sale and also implies that a different business entity has copied
or reproduced this model at some point in time.
A firearm having a barrel design in which the major length of the barrel is smooth and the last 10 to 12
centimeters are rifled.
A means of ignition of a propellant charge by a mechanical blow against a primer or percussion cap.
A loaded or unloaded firearm that is 30 inches or less in length, or a loaded or unloaded firearm
that by its construction and appearance conceals it as a firearm.
As it refers to firearms or firearms ammunition, a chemical, low-explosive mixture or composition that
when ignited burns rapidly (deflagrates), generating large volumes of gas (to propel a projectile).
A magazine-repeating firearm design that is manually actuated in motion usually parallel to the barrel.
Pump Action firearms are also sometimes referred to as slide action or trombone action.
Method of operation of semiautomatic and automatic firearms in which recoil energy imparted to
the barrel is used to actuate the mechanism to complete the cycle of operations.
A firearm with a mechanism for feeding ammunition into the chamber known as magazine
or belt that accomplishes the entire cycle of operation, either manually or externally
powered, or by utilizing some of the power generated by the propellant gas or recoil force.
Some of these mechanisms, externally powered systems or hand operated systems of
1. Pump action mechanisms
2. Lever action mechanisms
3. Bolt action mechanisms
4. Auto-loading or semiautomatic mechanisms; recoil-operated (both long and short
recoil) and gas-operated mechanisms
5. Automatic mechanisms; recoil operated (both long and short recoil) and gas operated
6. Manually powered, hand-operated, rotary-barrel mechanisms (Gatling gun)
7. Externally powered mechanisms (Vulcan Gatling or Hughes Chain gun)
The modern manufacture of any firearm that has ceased to have patent protection and is usually
no longer manufactured by the original maker. Normally the reproduction is equal to or better in
quality of material and workmanship than the original and in some cases may be an exact
duplicate with interchangeable parts and components.
A firearm, usually a handgun, with a cylinder having several chambers so arranged as to rotate around
an axis and be discharged successively by the same firing mechanism
A system of helical grooves cut into the bore of a firearm barrel. The purpose of rifling is to impart
spin to a projectile passing through the barrel, thus giving the projectile gyroscopic stability during
its flight to the target.
The receiver, which is also known as the frame, is the basic component of a firearm to which all
other components are assembled or attached. In most firearm designs the barrel is assembled to
the receiver and the receiver houses the breech mechanism, trigger and firing mechanisms.
A firearm designed or redesigned, made or remade, and intended to be fired from the shoulder and
designed or redesigned and made or remade to use the energy of the explosive in a fixed metallic
cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger.
Any type of applied safety catch. A mechanical device that enables the shooter to lock the firing
mechanism of a firearm voluntarily. Also called a manual safety or manually operated safety
catch. (NOTE: On most firearms with a fully-automatic fire capability, the fire selector usually
incorporates the safety catch.
A series of actions, verifications and precautions that must be undertaken before, during and after handling
a firearm or during firing of firearms to prevent accidents and personal injury.
An automatic device that prevents firing a firearm as long as certain conditions are not met. A
feature incorporated into the firearm's design to ensure that the weapon cannot fire, however
mishandled, before the breech is properly locked; and that the breech will not unlock until such
time as the projectile has left the barrel and pressures generated to expel the projectile have
dropped to a safe operating level.
Part of the firing mechanism, linked to the trigger, that retains the hammer or striker in the cocked
position until the trigger is pulled.
1. In double-barreled firearms, a device to allow the shooter to choose which barrel is
to be fired by the first pull of the trigger.
2. A lever that enables the shooter to choose the type of fire (full-automatic/semi-
automatic) or apply the safety catch; in the case of some automatic weapons, the
selector enables the shooter to choose high or low cyclic rate of fire.
A repeating firearm requiring a separate press of the trigger for each shot fired, using the energy
of discharge to perform a portion of the operating cycle. Semiautomatic actions are sometimes
referred to as auto-loading or self-loading actions.
A combination of numbers and letters applied to a firearm in order to uniquely identify it from all
others of its type. There is a requirement for the serial number to contain a minimum of one digit in
the combination of letters and numbers that comprise the serial number.
Plastic or paper insert surrounding the shot charge in a shot shell to reduce distortion of the shot
when passing through the barrel.
A firearm designed or redesigned, made or remade and intended to fire from the shoulder and
designed or redesigned and made or remade to use the energy of the explosive in a fixed shotgun
shell to fire through a smooth bore either a number of ball shot or a single projectile for each single
function of the trigger.
Pellets of various sizes used in shot shells and shot cartridges.
A rear sight for rifles that has the “V” notch at the bottom of an almost completely closed V.
A rear sight on pistols or revolvers having a flat top with a V-shaped notch used with a bead upped front sight. Named after Ira Paine, a 19th to -century exhibition and competition shooter.
A rear sight on pistols or revolvers having a flat top with a square notch used with a broad
flat-topped front sight. Named after E.E. Patridge.
A type of front sight of triangular appearance. Also called a Barley-Corn sight.
The distance between the rear sight and the front sight on a firearm.
A front sight mounted on a ramp base.
Any sight mounted on the upper tang of a firearm.
A rear sight in which the blade is adjusted for elevation correction by sliding along a curved cam or
cams. The term is also used for vertically standing back sights with sliding rear apertures.
1. ADJUSTABLE - Usually taken to mean a rear sight that is adjustable for windage
or elevation or both. Occasionally adjustable front sights may be found on pistols or
2. APERTURE - A form of metallic sight, front or rear, containing an aperture or disc
with a hole. Also called a Peep Sight.
3. BEAD - A form of front sight, usually found on shotguns, having the appearance
of a round bead.
4. EXPRESS- A series of rear sights, mounted on one base, with folding sight
leaves to be used for varying ranges or elevations.
5. FIXED - Metallic sights that are not adjustable.
6. GLOBE - A form of front sight usually used on target rifles, generally cylindrical in shape
and often arranged to accept various inserts containing either rings or posts.
7. HOODED - A front sight that is provided with a cover to shade it from direct light.
8. TELESCOPIC - A sight containing optical elements that magnify the target.
9. VERNIER - A long-range sight with a mechanism for adjusting windage, elevation
or both, as read on a scale in conjunction with graduations on a longer, linear
A device capable of attachment to the barrel of a firearm designed or intended to reduce the
noise of discharge of ammunition
A single-barrel firearm design that must be manually loaded, having no internal or external
magazine feed device. Certain single-shot firearms are operated by levers and certain single-shot
firearms are operated by a bolt. Regardless of the method of operating the breech mechanism,
these firearms are single-shot firearm actions.
An action that requires the manual cocking of the hammer before pressure on the trigger releases
the firing mechanism.
Hand-operated, hand- or shoulder-controlled firearms, including machine guns, up to and including
.50 caliber. It is generally accepted that firearms in excess of .50 caliber up to and including 30mm
caliber are called cannon, while firearms in excess of 30mm are artillery.
A mechanical device used to store energy.
A component, usually wood, to which a barreled action is attached, enabling the shooter to hold
the firearm and control it from the shoulder.
A firearm design that, as originally designed and manufactured, has the capacity of firing
projectiles in rapid succession during one press of the trigger, whether or not it has been altered
to fire only one projectile with one such press. In general terms a submachine gun is a compact,
short-barreled, air-cooled, magazine-fed firearm, usually chambered for a pistol cartridge.
The simultaneous firing of two or more cartridges chambered in the cylinder of a revolver, only one
of which is in exact alignment with the barrel; normally associated with percussion revolvers.
A rifle or shotgun in which the barrel and sometimes the magazine and fore-end are designed to
be removed from the receiver readily, without the use of tools, for the purpose of making a more
compact item for ease of transportation. When additional barrels are available for a take-down
firearm, these barrels are considered accessory barrels. See definition of accessory barrels.
A term used to describe a single-shot, multi-barrel or revolver mechanism on which the barrel or
barrels are allowed to tip down at the muzzle exposing the chamber or chambers for loading,
unloading or extraction of cartridges.
The lever located on the top of the breech end of hinged-frame firearms that when pushed to one
side will unlock the barrel(s) from the receiver or frame and allow them to move (left, right, forward,
rearward, tip-down or tip-up).
That part of a firearm mechanism that is moved manually to cause the firearm to discharge.
TRIGGER , RELEASE
An unconventional mechanism in which the firearm is fired by the release, rather than by the pull
of the trigger.
Either a single or double trigger arrangement on which the required trigger pull force can be made
very light by means of a "setting" mechanism.
A stage in the cycle of operation of a firearm in which the bolt, breech block or standing breech is
removed from fixed relationship with the chamber, so as to permit the extraction and ejection cycle
to take place after pressures have dropped to a safe operating level.
A four-barreled shoulder arm that usually has two smooth-bored and two rifled barrels.
The flat portion of the receiver or frame on firearms that break open, extending forward from, and
is approximately at right angles to, the standing-breech face. This is the surface on which the
barrel flats rest when the action is closed.