Reviewed/updated 28Feb2023
| Action | The mechanism by which the firearm cycle of operation – from firing to reloading the next round – is accomplished. The most common types of action are semi-automatic, bolt, lever, and pump. | |
| Automatic | A self-loading firearm that maintains a rapid continuous rate of fire as long as the trigger is depressed | |
| AR-15 | Armalite Rifle model 15; a lightweight select-fire (semi-automatic or automatic) rifle designed and originally manufactured by Armalite and subsequently licensed to Colt’s Manufacturing Company. Civilian versions lack automatic fire capability. | |
| Assault rifle | There currently is no universally accepted definition of “assault rifle”. | |
| Bolt-action | Action in which a handle attached to the bolt (cylindrical device) is used to rotate (unlock) the bolt, which is then pulled rearward to extract a spent case from the barrel. Moving the bolt forward loads the next cartridge into the barrel and rotating (locking) the bolt readies the gun for firing. | |
| Breech | The rear end of the barrel attached to the action | |
| Bullet | The projectile that is driven from the immobile cartridge case upon ignition of the powder within and exits the barrel at high speed; usually made of lead and often with a thin copper coating | |
| Bump stock | A device which uses recoil energy to enable a semi-automatic firearm to maintain a continuous rate of relatively rapid fire | |
| Caliber | Internal diameter of the barrel bore, usually expressed in inches (e.g., .22, .45) or millimeters (e.g., 9 mm) | |
| Carry reciprocity | Agreement between states or other jurisdictions allowing the owner of a firearms license or permit in one state or jurisdiction to legally carry a firearm in another | |
| Cartridge | Ammunition that comprises a case, gun powder, a primer or priming compound, and a bullet | |
| Case | A metallic cylinder containing gun powder, with a primer or priming compound at the bottom and into which the bullet is inserted at the top | |
| Centerfire | A cartridge which contains a discrete primer that initiates gun powder ignition | |
| Chamber | The opening at the rear of the barrel designed to hold a specific cartridge | |
| Class 3/NFA firearm | Title II NFA (National Firearms Act) firearm whose ownership requires an application and a tax stamp to be paid when purchasing the firearm (e.g., machine gun, short-barreled rifle or shotgun, suppressor) | |
| Clip | A device used to load multiple rounds into an internal magazine of a rifle, or into the cylinder of a revolver | |
| Compensator | A device integral or attached to the end of a barrel primarily to reduce muzzle rise upon firing by redirecting expelled gasses; may also reduce recoil | |
| Concealed Carry | Carrying a firearm in a manner that generally prevents it from being visible to others | |
| Constitutional carry | The recognition that the U.S Constitution represents an individual’s permit to carry a handgun in public without additional permits. Maine, New Hampshire, and Vermont are constitutional carry states. | |
| Cycle of operation | Steps in an action that include firing, unlocking the action, extraction of the empty case from the chamber, ejecting the case from the firearm, cocking, insertion of a new cartridge into the chamber, and locking the action | |
| Flash hider/flash suppressor | Device at the end of a barrel that disrupts expansion of hot gasses from the barrel upon firing, minimizing visible “flash” from the escaping gasses | |
| Gauge | Internal diameter of a shotgun barrel, based on the weight of a lead sphere that will fit into the barrel and expressed as the fraction of a pound; e.g., a 12 gauge barrel will fit a lead sphere weighing 1/12th of a pound | |
| Ghost gun | A firearm built by an individual from parts that include at least one non-functioning component that requires modification by the builder to become functional. Parts can be purchased without a background check, and the firearms built are not required to have serial numbers. | |
| Hammer | A pivoting device that strikes the rear of a cartridge or a firing pin to initiate ignition of gun powder in the cartridge | |
| Handgun | A relatively short-barreled firearm that can be used with one hand | |
| Lever action firearm | Action based on a downward/upward pivoting lever below the receiver which is operated by the shooting hand and enables the cycle of operation | |
| Loaded | (vs. unloaded) A firearm in which ammunition is readily available for firing; e.g. a cartridge is in the chamber or cylinder, or in a magazine inserted into the firearm | |
| Magazine | A removable or integral device for storing ammunition and feeding it into the firearm | |
| May-issue permit | A concealed carry permit that may be issued upon application, at the discretion of local law enforcement officials. Massachusetts is a may-issue state. | |
| Modern sporting rifle | Semi-automatic AR-15 style rifle | |
| Muzzle | The forward end of a barrel from which the bullet exits | |
| Muzzle brake | Device integral or attached to the end of a barrel primarily to reduce recoil upon firing by redirecting expelled gasses; may also reduce muzzle rise | |
| Open carry | Carrying a firearm in a manner that does not prevent it from being readily visible to others | |
| Pistol | Handgun | |
| Pistol grip | A vertical or near-vertical handle under a firearm that is gripped by the support hand | |
| Pump-action | Firearm action based on the backward/forward motion of the forearm of the firearm to enable the cycle of operation. “Pump-action” is generally used in reference to shotguns, while “slide-action” is used in reference to rifles. | |
| Receiver | The physical housing for the action of a firearm | |
| Recoil | Rearward movement of a firearm following discharge | |
| Red flag law | Allows members of limited groups (family members, law enforcement) to petition for a court order requiring an individual who is considered a danger to themselves or others to temporarily surrender their firearms to law enforcement. A subsequent hearing with the defendant present would determine whether to return the firearm(s) or to extend the order. Petitioners making false claims would face criminal charges. Red flag laws are also widely referred to as an extreme risk protection orders. New Hampshire does not have a red flag law; Massachusetts does. | |
| Revolver | A handgun which uses a rotating cylinder (containing multiple cartridges) to align cartridges with the barrel prior to firing | |
| Rifle | A firearm with a relatively long spirally grooved barrel and a stock which provides stability when pressed into the shoulder or used with another support | |
| Rimfire | A cartridge which contains primer compound in the rim of the casing (instead of a discrete primer in centerfire cartridges) that initiates gun powder ignition | |
| Round | Cartridge | |
| Safety | Mechanical devices of any of a variety of designs which, when engaged, are intended to prevent the accidental discharge of the firearm | |
| Semi-automatic | A self-loading firearm that requires the trigger to be manually depressed for each shot; thus, each trigger pull results in the discharge of a single round | |
| Shall-issue permit | A concealed carry permit that is generally issued upon application, provided that the applicant meets minimum legal requirements. Maine and New Hampshire are shall-issue states; Vermont does not issue permits. | |
| Silencer/suppressor | A device integral to or attached to the barrel of a firearm which reduces (does not eliminate) the sound and recoil resulting from discharge by redirecting expelled gasses | |
| Shotgun | A firearm with a relatively long, smooth-bored barrel(s) which fires shotshell cartridges containing numerous individual pellets (shot) or a single slug. Barrel diameters are expressed in gauge units (e.g., 12 ga). | |
| Stock | A wooden or synthetic device attached to the back of a receiver which aids in aiming and stabilizing the firearm when firing | |
| Striker | A spring-powered firing pin that strikes the primer in centerfire cartridges when the trigger is depressed | |
| Transfer bar | A safety device that, when the trigger is pulled, moves into position to engage the primer of centerfire cartridges | |
| Universal background check | A system under which a prospective firearm recipient would be required to undergo a background check using the National Instant Criminal Background Check System (NICS) prior to ANY firearm purchase or transfer. No universal background check system is currently in place in the U.S.; only sales by Federal Firearms License (FFL) holders now require background checks through NICS. |