Ammunition

Ammunition is a catch-all term for what ever is actually fired from a ranged weapon, or thrown as a weapon itself.

Modern Firearms
Modern Firearms use cartridges commonly referred to as "bullets" as ammunition. Most firearms are developed to use only one particular kind of bullet, and when I say that, I'm not saying that they are merely restricted to a single caliber, I mean most guns can only fire an extremely small selection of bullets.

Anatomy of a Firearm
There a multiple different kinds of firearms.

Parts
Permanent parts include:
 * The grip - where the gun is held.
 * On long guns - a front grip, where the weapon is stabilized.
 * A trigger - used to fire the weapon.
 * The trigger action - the mechanism that the actual trigger "triggers".
 * On single action or single and double action weapons, an external cock to arm the weapon.
 * The load-action, what ever mechanism is installed for re-loading the weapon.
 * Chamber - where the bullet is primed for fire.
 * Magazine - be it an internal magazine, external or a revolving drum, magazine i the term used to refer to where the bullets are stored in a gun.
 * Sights - what is used to aim.
 * Stock - typically only found on Long guns.

Gun types
There are various kinds of guns - seen best as overarching categories. They all use different types of ammunition, commonly.

Utility weapons
Utility weapons are not meant for mobile use, but are mounted permanently or temporarily on a given object to provide stability. They often function very different in their inherent mechanics from ordinary firearms. Utility weapons normally fire special ammunition only used for utility weapons - such as launch-able slugs (often grenades), belt-fed ammunition and many other nifty things that aren't very practical for a single lone shooter do have to deal with.

Long guns
Long guns describes weapons that are intended exclusively for two-handed use. Long-guns use ammunition that is typically much longer and bigger in size compared to Pistol Ammunition. Both chamber and projectile of the bullet is commonly MUCH longer, - one notable exception being the .22 Long Gun, but this is hardly a combat round.

Rifles
The word "rifle" refers to a long gun that has rifling in the barrel, and rifles replaced the muskets that were smooth-bore (no rifling).

Carbine
Interestingly, carbines have changed in their meaning over the last few decades due to the invention of the sub-machine gun. Originally, a Carbine was a rifled long-gun that was significantly shorter than a typical rifle so that it could be fired from horseback with one hand if necessary, and fired pistol rounds with much greater stability than a pistol could. In modern times, with the invention of the sub-machine gun half-way filling the same niche as the original carbine, the carbine became a compact rifle instead, recognized by it's short barrel and somehow collapsible or foldable stock.

Shotgun
A shotgun is for all effects and purposes, a very wide-barreled musket that takes a modern cartridge. It's a smooth-bore rifle, essentially. Just stop and think how ridiculous the word "shotgun" is for a moment. Yeah, that's right. A smooth-bore pistol is also a shotgun by definition - though in most cases, these are referred to as "launchers" and are used for slightly different purposes.

Pistol
Pistols are, unlike long guns, much smaller and more easily carried without being obtrusive as well as concealable. That said, they are not necessarily designed for one-handed use, nor even necessarily recommended for one-handed use, but the first pistols WERE intended for one-handed use, wielding a melee weapon in the other hand.

Derringers and other uncommon pistol designs
Today, very few pistols do not either fall into the revolver or auto-loader category - the derringers are commonly found in this category, being break-action pistols, but there are all sorts of strange pistols out there.

Revolver
The revolver is one of the earliest designs of pistol to still see common use today. Revolvers actually have a number of interesting advantages over their more modern auto-loader brethren, such as significantly less recoil and drastically improved reliability due to the almost foolproof design.

Auto-loader
Auto-loader pistols are the kind that reload themselves by pulling a new bullet from a magazine. These pistols are tactically the most practical due to fast reload times, generally less trigger resistance and faster rate of fire, but the recoil is also heftier and they are more prone to jamming.

Assault weapons
Assault weapons are long guns or pistols that have the ability to rapid-fire (also known as automatic fire), meaning that, not only is the weapon auto-loaded, but it also has a firing mode where the weapon continuously fires when the trigger is held in the pulled position.

Sub-machine gun
A sub-machine gun is an assault pistol or an assault carbine that fires pistol ammunition instead of long-gun ammunition. The former is commonly called a micro sub-machine gun, where as the latter is a normal sub-machine gun.

Load Actions
Definitely a confusing and archaic term, "action" refers to the action that happens when the gun is reloaded.

Muzzle loader
A muzzle loader is not at all type of action, but is included here for the purpose of clarity. Some, mainly archaic rifles, pistols and shotguns, are front-loaded. This kind of loading a weapon predates having any kind of actual load-action.

Break-action
The first kind of "action" guns, are guns that are reloaded by releasing a small clamp that opens the chamber and allows manual insertion of a new bullet. Break-action is common in archaic rifles, oddity pistols and very common with shotguns.

Bolt action
Bolt action is a gun that either functions as detailed above, by opening the chamber by pulling back on a bolt, thus allowing manual insertion of a bullet OR causes the weapon to reload from a magazine once this action is taken. Bolt action is exclusively used for rifles and a select few (and very strange) pistols.

Autoloader
An auto-loader weapon is a weapon that reloads on it's own after firing, provided there are any bullets left in the magazine, be it internal or external. Such weapons are typically technically bolt-action, as the weapon is initially loaded using a mechanism very similar to a bolt, that opens the chamber, though more modern firearms use very untraditional setup for this, with a pulling handle on the back of the rifle.

Slide-action
A slide-action auto-loader is the most common type of pistol auto-loader action - the weapon is loaded by pulling back on a slide.

Lever action
Lever action was invented for carbines, to allow single-handed reloading, and always makes use of a magazine - usually an internal magazine, but it also became popular with use of shotguns.

Pump action
Similar to bolt action, in place of a bolt that opens up the chamber, there is a pump handle on the gun that is pumped to reload from an internal magazine or an external magazine. Popular almost exclusively with shotguns.

Trigger action
Trigger action is different from other "action types" as mentioned above, as it has nothing to do with reloading, but with the arming of the gun rather. Trigger actions come in three different varieties; Most rifles are single action, and single action is also not uncommon among modern auto-loader pistols - whether something has one kind of trigger action or not, is ultimately only really important with old auto-loaders or revolvers.
 * 1) Single action - where the trigger serves one function - to fire the already primed bullet.
 * 2) Double action only, where the there is no external cock that can be used to arm the gun, and pulling the trigger arms the weapon and fires in the same go.
 * 3) Finally, there is single- and double action, where the gun can be manually cocked and fired, or where pulling the trigger will do both if the gun is not already armed.

Anatomy of a bullet
Just like a gun, the bullet itself also consists of multiple parts.
 * Projectile - the tip of the bullet, that is fired - with shotgun shells, the projectile(s) is often entirely concealed inside the casing. Also commonly called a "slug".
 * The large (normally brass) part of the bullet is the casing - it is full of gunpowder and other chemical compounds that cause the desired reaction when the trigger is pulled.
 * Primer - the essential "detonator", that when struck, results in the projectile being fired from the casing

Types of ammunition
There are two over-arching types of ammunition:
 * Pistol Ammunition - generally high-caliber bullets, the smallest combat calibers still being larger than most large caliber rifle bullets by comparison, but the bullets are short and commonly rounded, intended not on penetration but meant to lodge in the body and reflect around on bones. The casing is also short, meaning not much gunpowder is in the round - the firepower is dramatically reduced. Some high-power pistol rounds come close to resembling long-gun ammunition in fire-power and slug-size.
 * Long-gun Ammunition - If one was to make a pistol round in caliber .40, then that pistol round would very likely have a much shorter casing and smaller slug than a comparative long-gun bullet in the same caliber.

Caliber
Caliber was originally an imperial measurement, and is the approximate internal diameter of the barrel, or the diameter of the projectile it fires, in hundredths or sometimes thousandths of an inch. Colloquially caliber is used simply to denote the size of a bullet or cartridge, regardless of what measurements they are given in, and as such it has come to mean the "size of the bullet or barrel of the gun", which often confuses people

European Caliber
In Europe they use metric caliber measurements, meaning they start with the length of the projectile itself in millimeters with at least two decimals followed by an X that denotes that the next number is separate from the first, the next number following the X being the length of the casing in millimeters, rounded up to the nearest number with no decimals. An example could be 5.56X45mm, read as "five point five-six by forty-five millimeters", even though this seems to indicate that the number is a flat area due to the formula, though this is not the case.

Certain European Calibers have become so common to refer to, that people neglect to use the full designation - such as referring to a 5.56X45mm round as a "5-5-6" or referring to 9X19MM round simply as "9 millimeter".

Shotgun Caliber
Shotguns are measured in "gauge", which is actually a completely different kind of imperial measurement. Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm, and is expressed as the multiplicative inverse of the sphere'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, etc. The term is related to the measurement of cannon, which were also measured by the weight of their iron round shot; an 8 pounder would fire an 8 lb (3.6 kg) ball.

The most important thing to remember here, is that due to the measurements above, smaller gauge means a bigger cartridge, where a 1 gauge is essentially a solid 1 pound lead-sphere in size, where a 20 gauge is the size of 1/20th of a the former mentioned lead-sphere.

Caliber versus specific cartridges
Most people make the wrongful assumption that a gun chambered in, say, .45 caliber, will fire any bullet that is also .45.

This is completely wrong however - first of all, a rifle may be capable of firing pistol rounds on the off-chance that they will fit in the chamber, without falling out or some such, but a pistol will generally never fire long-gun rounds.

Long-gun rounds are longer than pistol rounds - having bigger chambers, and typically are smaller caliber than pistol rounds. The avid physicist probably realizes that this means that pistol rounds are fairly powerful in short distance, but have a very fast bullet drop, where as long-gun rounds will have a much greater firing distance and superior penetrative power.

Further problems include that various weapons are intended for a certain chamber pressure and that the dimensions of certain bullets, despite being the same caliber, may not have exactly the same dimensions. It's for instance very likely that you could fire a .45 Glock Automatic Pistol round from pistol designed to fire .45 Automatic Colt Pistol rounds, but there is no guarantee. however, you could most likely NOT do the reverse, because the .45 ACP is a longer round than the .45 GAP, and as such it would likely not even fit in the chamber.

Neither Caliber nor gauge accounts for the overall length of the bullet, European Caliber does though - resulting in much less confusion, which is also why any given military will commonly use European Caliber.

Primer types
There are two general primer types: Rim-fire primers are primarily used with fairly weak projectiles used for target practice and vermin hunting. There are some slightly more powerful variants that can be used for bigger game as well, but they are uncommon. Due to the primer method, the casing has to be thin for rim-fire to work, meaning that the casing cannot endure large pressure - and as such there is an upper limit to how powerful a bullet that is rim-fire can essentially be.
 * 1) A center-fire primer (by far the most common), where the primer is dead-center on the back of the bullet.
 * 2) A rim-fire primer, where the primer lays in a ring on the edge of the back of the bullet, fired when something hacks into the rim.

Center-fire suffers from no such limitations.

Magnum and other terms
Magnum means "large" or "big" in Latin, and when magnum is attached to the name of a bullet, it means that the particular bullet has a larger than normal casing - meaning more firepower.

It's very important to remember again, that just because a gun is stated as being compatible with a certain caliber, it doesn't mean that it can necessarily handle any given length or chamber pressure for that matter, so the gun must specifically be compatible with magnum rounds of the particular caliber. Magnum isn't commonly added to caliber in it's own - it's added to a specific cartridge name.