Let’s see what I can do. Fair warning, there’s probably going to be a few very minor technical inaccuracies. I’m typing this off the top of my head.
Gun: pretty much any gunpowder based weapon. This includes both hand weapons and artillery. It does not (normally) include weapons that fire self-propelled ordinance, such as a missile launcher.
Gunpowder: This is actually a catch all term. Early gunpowder (now called “black powder) was mixed from saltpeter (potassium nitrate), charcoal, and sulfur. Modern firearms use variations of smokeless powder, originally developed in the late 19th century. Black Powder is still used with some antique and replica weapons.
As an academic distinction, it’s worth pointing out that gunpowder isn’t actually explosive, it just burns very aggressively, which results in the expansion of gas and bushing the bullet into motion. Unburned powder that remains in the gun is a persistent headache in gun design, and why guns need to be cleaned frequently.
A tiny amount of an explosive, called the Primer, is used to get the powder burning. Historically this has included substances such as picric acid and nitroglycerin. I believe modern primers use lead styphnate, but I’m honestly not sure, off the top of my head.
Cartridge: The entire package of a bullet, powder, and primer. In modern weapons, the container itself is referred to as a shell casing. The shell casing can also be referred to as a shell or casing, independent of the other.
Shell casings are sometimes referred to, idiomatically as “brass” because it is the most common material, though aluminum and other soft metals are used.
Idiomatically, shotgun cartridges are referred to as shells. Historically these were frequently made from paper. Modern shells use corrugated plastic, color coded to denote the contents.
After having been fired, a cartridge (or shell) is referred to as “spent.”
Firearms/Small Arms: Firearms, primarily refer to handheld gunpowder weapons. Small Arms refer to guns with a bore diameter (literally the size of the barrel) smaller than an inch.
Bore: This is the literal hole in the center of the barrel, that the projectile(s) travel through.
Chamber/Battery: Both terms are technically correct. This refers to where the bullet resides when the weapon is ready to fire. If a weapon’s chamber is empty, it is impossible to fire it.
Chambered: Both the state of a round being in the chamber, ready to fire, and a term that refers to the cartridge size a firearm has been designed to accept. Examples: “Chambered in .308.” “It has a round chambered.”
Incidentally, “rechambering” a weapon refers to changing the rounds a weapon will accept, removing a round and loading a fresh one is cycling (see below). Rechambering a weapon usually involves replacing the barrel, action, and magazine. Though it can be more involved if alternate parts aren’t available.
Action: The mechanical systems that clear and replace the bullet in the chamber whenever the weapon is fired. We’ll come back to this with a couple varieties in a bit.
Cycle: The actual process of the action functioning. Depending on the firearm, this can either occur automatically with each trigger pull, or it can require a direct user input.
Receiver: The physical housing that holds the action.
Hammer: a physical component behind the pistol which strikes the firing pin. Not all firearms have one.
Bolt: This is the component that actually locks the cartridge in the chamber, when the weapon is ready to fire.
Firing Pin: This is a small metal cylinder pin which (in modern firearms) strikes the back of the cartridge, detonating the primer, igniting the powder. Usually this is a separate articulated component, though some weapons have a simple stud soldered onto the bolt.
Open Bolt/Closed Bolt: This refers to which configuration the weapon fires from. Technically the bolt needs to be closed in order to actually get the bullet moving.
With an open bolt design, the act of dropping the bolt will detonate the primer. This is primarily used with fully automatic weapons (see below). The bolt will fall, igniting the primer, recoil will send the bolt back, and the return spring will cause it to close, firing again.
Magazine: This is where rounds are stored inside of the weapon, before firing. The action will extract a round from the magazine each time it is cycled. Depending on the firearm, this may be removable.
Clip: A device used to load a firearm’s magazine. This is NOT interchangeable terms. Usually these are short metal strips that grip the base of the shell, though speedloaders for revolvers sometimes qualify as clips.
Clips can be used with modern weapons to quickly reload box magazines, but they’re somewhat uncommon.
If the firearm’s reloading process involves loading the rounds and then ejecting the clip, that’s, well, a clip. If the reloading process involves removing an empty container, and loading a fresh one, that’s a magazine.
Few things will irritate someone with firearms training faster than mixing these terms up.
Rifles: The term actually refers to two separate things. Rifling are mildly spiraling lands and grooves cut into the barrel of a gun. This prevents the bullet from tumbling once it leaves the barrel, and massively improves accuracy. Rifles originally referred to any firearm that featured a rifled barrel. However, the term is no longer inclusive, because handguns and other non-rifles include rifled barrels.
Handgun: A smaller version of a firearm that can be operated with one hand. This term is inclusive of several different varieties of firearm. It should be noted: you should not use a handgun one handed, but it is possible.
Pistol: This refers to nearly every handgun, except revolvers.
Revolver: A firearm that rotates to feed rounds into the chamber. Most often this refers to handguns, though some grenade launchers also use a revolver design. Revolver rifles, carbines, and shotguns exist, but are rare. There is a small gap between the cylinder and the barrel, which tends to vent burning powder when fired, which makes revolvers with a fore grip unpleasant to use. That is to say, they’ll try to set your shirt or arm on fire.
Shotgun: This refers to a weapon designed to handle unusually large cartridges, holding multiple projectiles. These are frequently smooth-bore (see below), but rifled shotguns do exist. In modern combat, shotguns are more characterized by their ability to accept a wide variety of projectiles to accommodate different situations. Shotguns can be loaded with everything from water (disruptor shells), to grenades (FRAG-12s).
Smooth-Bore: A barrel without any rifling. Most common with shotguns. This favors projectiles that will somehow self stabilize (such as flechette darts, yes, it’s another shotgun shell variety), or fire multiple simultaneous projectiles (such as a shotgun).
Single Shot: This refers to a weapon that can be fired once, and then must be reloaded. This includes muskets and some shotguns.
Semi-Automatic: A non-revolver firearm that will fire a round with each pull of the trigger until the magazine is depleted. Critically, the weapon must do this automatically as a result of firing. If a weapon needs to be manually cycled, such as a bolt or lever action, it is a repeater, and not semi-automatic. In any case where “automatic” is paired with another word, it can be abbreviated as “auto.”
Automatic: A firearm that will fire multiple rounds with each pull of the trigger. Also sometimes referred to as Fully Automatic. Idiomatically, semi-automatic pistols are sometimes referred to as “Automatics.” This is incorrect on a technical level, but the actual meaning is, usually, understood.
Burst Fire: An automatic firearm that fires a specific number of rounds with each pull of the trigger and then stops. Three round burst settings are the most common, though two round burst weapons have proven popular in some circles.
Select Fire: An automatic firearm that can be switched between multiple fire configurations. Most often this allows switching the weapon between semi-auto and full auto, or semi-auto and a burst fire setting. Select fire almost always includes a semi-auto setting. It can include multiple other settings, including (rarely) both 2 and 3 round burst settings.
Single Action: A firearm where pulling the trigger will not cock the hammer. This is intermittently used as a safety feature on modern handguns. It is also somewhat common among sport revolvers, and antique revolvers.
Single Action firearms often have a much lighter trigger pull (the force needed to draw the trigger and fire the weapon). This allows for greater accuracy. It also allows automatics to be carried with a round in the chamber and the hammer down, without risk of the weapon firing as a result of an errant trigger pull. It’s still shouldn’t happen with safe weapon handling, but it is another safety feature.
Double Action: A firearm where pulling the trigger will cock the hammer. Almost all revolvers intended for practical use include this. It’s inclusion with semi-automatic pistols varies widely.
The complicated issue with single and double action handguns comes from semi-auto pistols. When the slide cycles, it will recock the hammer, this means a single action pistol can be fired repeatedly, without having to manually recock the hammer.
SAO/DAO: Single Action Only/Double Action Only. These terms get applied to pistols because there are pistols designed to switch between single and double action based on a variety of control parameters.
For example: pulling the slide back ~1/4″ on a Walther P99 will switch it from single to double action, and vise versa. Though it also exists in SAO and DAO variants that remove this feature and lock the action in one of the modes.
Bolt Action: A firearm where the bolt must be unlocked and manually cycled by the user. This allows for substantially heavier loads than any other style of firearm. Though it is a popular configuration for hunting and varmint rifles.
Lever Action: A firearm where the action is cycled by use of a lever, usually mounted under the handgrip. Originally these allowed for faster cycling than a bolt action weapon. These are fairly uncommon now.
Pump Action: A firearm that cycles the action by use of an articulated foregrip. This is normally seen on shotguns, though a few pump action rifle models probably exist.
Machine Gun: This refers to a fully automatic weapon. By itself the term is antiquated. Most often, when someone uses the term, they’re incorrectly referring to an Assault Rifle.
Assault Rifle: A select fire weapon chambered in an “intermediate” rifle round. Usually between 5mm and 6mm. Note: these do not always include full auto settings. The modern M16 variants can only be fired in semi-automatic or 3 round burst.
Battle Rifle: A select fire weapon chambered in a high power rifle round (roughly 7.6mm). This includes the M14 and AK47/AKM. Misidentifying a battle rifle as an assault rifle is… eh. It happens.
Carbine: A shortened rifle. Usually assigned for use in tight quarters, or vehicle crews. Historically these were also issued to cavalry. Sometimes issued to support personnel, depending on the military.
Light Machine Gun: Also sometimes referred to as a Squad Support Weapon is an unusually heavy automatic rifle intended for use in suppression. Sometimes abbreviated LMG.
Submachine Gun: An automatic weapon chambered to fire a pistol round. Sometimes abbreviated SMG.
Machine Pistol: A submachine gun that retains an overall pistol design. Informally, these terms can get mixed up pretty heavily.
Caliber: This is the imperial system of measuring bullet diameter. It’s expressed as a period with a two digit number. (EG: .45 or .38) This indicates the size of the cartridge in 100ths of an inch. So .50 is, roughly, half an inch in diameter. Additional digits beyond the first two denote differences in the cartridge, but not significant changes in the cartridge size. (EG: .308, .303, 30.06 are all .30 caliber rounds, roughly.)
Gauge: The imperial system for measuring the size of a shotgun shell. This one’s a little more idiosyncratic. It’s calculated based on the weight of a solid ball of lead, that barrel would accommodate. So 12 gauge will fit a single 1/12th pound ball of lead. This also means, as the gauge goes up, the size shrinks. 20 Gauge shells are significantly smaller than 12 gauge, for example. This is abbreviated as “ga”, so “410ga” would indicate a 410 shell.
Millimeter (mm): The metric system for measuring the size of a bullet. Usually expressed as a simple value. (EG: 9mm or 5.56mm). When multiple cartridges exist that are of similar sizes, other terms will be applied. (Technically, this also occurs with calibers. For example: .357 Magnum, and .357 SIG.) With metric measurements, the length is frequently added to distinguish two similar rounds, (for example: 9x19mm vs 9x18mm) or some other distinguishing characteristic. (for example: 9mm Parabellum vs 9mm Makarov). Usually you do not need to include both together. For example: 9x19mm Parabellum would be redundant, because 9mm Parabellum is a 9x19mm round.
Grain: The amount of powder loaded with a bullet. (Literally, an archaic unit of measurement.) Bullets in a specific caliber are usually available with multiple grain variants. For example: .45 ACP is commercially available anywhere from 185 grain to 230 grain.
Handload: The act of manufacturing your own bullets. Also a term for non-standard rounds produced this way.
Load: A term for the individual characteristics of a round that go beyond the size of the bullet. This includes the grain, and may include the kind of bullets (see below).
Magnum: A term denoting an unusually high grain load. Most commonly associated with the .357 Magnum and .44 Magnum rounds. Though other magnum rounds exist.
Ball: A bullet with a rounded tip. The most common kind of ammunition for handguns.
Hollowpoint: A bullet with a divotted tip. On impact, it causes the bullet to expand flattening. In a human body, this can sometimes sheer apart, and can cause catastrophic internal damage.
While illegal, an individual can add a small high explosive to the tip of a hollowpoint round, converting it into an improvised high explosive round. The most commonly available materials that would react appropriately are primers.
Wadcutter: A bullet with a flat tip. Usually employed in target shooting, to create clean holes in targets.
[Material] Jacketed: Frequently copper, though other soft metals are sometimes used. This is used to partially shield the user from the bullet’s lead, and the associated health risks.
[Material] Core: Most often, the material is steel, though spent uranium (in this case, spent is a nuclear term, not the firearms meaning), is an exotic variant. The core will push through materials that would stop normal bullets. Lead shields the core from the barrel. (Firing a steel slug from a firearm would shred the rifling, so the softer metal contacts the metal.)
Tracer: A pyrotechnic round that ignites on contact with air and shows the shooter exactly where the round went. These are also mildly incendiary, and can start fires if they connect with something flammable on the other end.
I’m not going to give a full list of what you can stick in a shotgun, because it’s a very long list. But, a few quick highlights.
Buckshot: Ball bearings, usually lead or steel.
Slug: A single, solid, bullet.
Flechette: A steel dart, usually with fins to stabilize it in flight. Fired with a plastic sabot system that falls away once the dart is in the air.
FRAG-12: A small, impact detonated grenade, designed to be fired from a 12 gauge shotgun.
Flares: Commercial flare guns fire a low power 12 gauge shotgun shell. While you cannot load normal shotgun shells into a flare gun (it’s not designed for that kind of power, and will explode), 12 gauge flare shells can be loaded into a shotgun and fired. If the shotgun is semi-automatic the flare will probably not provide enough force to cycle the action, so the user will need to do that manually.
Dragon’s Breath: A shell loaded with a mix of oxygen igniting metals. Metallic Sodium and Potassium are most common. This creates the effect of the shotgun blasting flames.
I feel like I’ve only scratched the surface, and I know I’ve missed a few things. I’ll try to remember to revisit this in the near future.
Hope this helps some of you get started.