I posted something about this a long time ago, but I guess I need to re-do it (and add in some reality on some other vocabulary). I just get tired of people not calling things what they should when it comes to guns, as its such a polarizing topic at this point.
Types of Action:
Full-Auto: Pull the trigger, empty all ammo available. Illegal for a non-FFL Class 3 License citizen in the US since 1986 (Correction as per Joel's comment below: 1968, but started to be regulated as early as 1934). As the average person doesn't get to use them without special legal requirements, its not really necessary for me to talk about it. I repeat, if you do not go through all the hoops of being licensed specially (paperwork, payment, essentially registration of your firearm), you cannot do this legally.
Semi-Auto: Pull the trigger, one bang. Pull the trigger, one bang. You can empty the magazine, but it takes one trigger pull at a time. Can hold anything from 15 on up, depending on the type of magazine (see below). Mostly its 10 and 30 for the most popular styles.
Pump Action: Usually the fore-end of the gun is on a slide, and you pull the trigger, pump the slide, next round in the chamber. These are often tubular magazines, and hold between 5 and 8 rounds.
Lever Action: Think John Wayne. Has a lever attached to the trigger guard, and when you work the lever, the next round enters the chamber (yes, this comes in a shotgun as well. Chiappa makes it). Typically from 4-7 rounds in the gun.
Bolt Action: This is probably the slowest, but most accurate of the actions. Essentially there's a handle that gets lifted and pulled back to eject a shell, then pushed forward and dropped so that another round is ready to fire. Typically this type of gun holds 3-6 rounds.
Single Shot: One bullet in, fire. Then you have to open the gun (Usually a break action, where the gun bends at the base of the barrel separating the firing action from the barrel), have the cartridge pop out, and put a new one in the barrel for the next shot to be fired.
Full Auto: Same as rifle, including legality.
Single Action: Pull back on the hammer, pull the trigger. There are semi-auto pistols that are single action, where they have to be cocked first, and then after each subsequent pull of the trigger, the hammer is returned to ready to fire.
Double Action: Pull the trigger, hammer pulls back, falls, gun fires. In a revolver, this works similarly, but with fewer shots as there isn't as much ammo in a revolver cylinder compared to a magazine.
Magazine- Holds ammunition. There are two basic types, detachable and fixed (fixed could be a fixed box magazine or a tubular magazine). The fixed tubular can hold (in some cases) as many as 16 rounds, depending on the size of the bullet/ammunition being used. However, it is slow to load. The detachable box magazine can be pre-loaded, dropped from the gun, and another loaded magazine put in. In pistols, the big difference is between revolvers (that have a cylinder) and the magazine style. This difference is speed of loading. However, if a person has speed loaders, the revolver can load almost as fast as the semi-auto, but it still lacks the number of rounds to be fired, as most revolvers are 5-7 rounds, and most pistols are 8+ (if you have the right magazine in the right semi-automatic pistol, this could be as high as 100. However, that magazine is awkward and useless for accuracy as it is ridiculously heavy and more of a toy than an effective accessory on a gun. That's my opinion, others may disagree).
Pistol Grip: This is a small piece of plastic in the shape of a handle placed either near the trigger guard or on the fore-end of the gun to give better grip. It looks like a more military style, but does nothing to change the function of the gun, unless the ergonomic improvement somehow increases accuracy.
Scope: Monocular, multi-lens (think half a binocular) with a crosshair inside to improve distance shooting. Can be a fixed magnification or a variable (adjustable).
Collapsible Stock: Allows for the stock to be adjusted, or moved for storage, shorter use. Can make the gun easier to conceal, but in many cases (specifically the AR style rifles), it allows for easy adjustment for shooters with different size bodies so more than one person can easily use the firearm.
Iron Sights: The old fashioned v towards the back and the pin towards the front. Can also refer to the bead on a shotgun, or any of the other styles of sights that are used without a scope. Can also include the fiber optic styles of sights.
The first big difference here is that "Caliber" refers to rifle and most pistol ammunition, while "Gauge" refers to shotgun and some pistol ammunition.
Shotgun Ammunition: Typically comes in a plastic shell casing. Can contain shot (small projectiles packed together, but when fired that spread into a pattern that gets wider with distance) or a single projectile called a slug (there are more specifics here, but not worth the discussion). A slug is essentially a 1 oz chunk of metal that, depending on the setup, can be accurate out to 150-200 yards. Shot, on the other hand, depending on its size, is usually only accurate out to 50 at the max. The higher the size of the shot, the smaller the size and greater the number of projectiles in the shell. The lower the size of the shot, the larger the size and fewer the number of projectiles in the shell. So, 9 shot is essentially lead flakes, where 00 Buck is more like 3-5 lead balls in the shell. The other thing with shotgun ammunition is that the smaller the gauge, the bigger the shell. So a .410 is smaller than a 28, a 28 is smaller than a 20, a 20 is smaller than a 16, and a 16 is smaller than a 12 (there are other gauges that go smaller, but only the 10 gauge is legal to produce any more, and that is a rarity).
Rifle Ammunition: Excluding special cases, this is typically a bullet with a brass or steel casing that fires one bullet for each pull of the trigger. Calibers are their own unique study, but the best way to comprehend them overall (as they sometimes defy mathematical common sense) is that the smaller the number, the smaller the bullet (not necessarily its impact, but the bullet size), as most calibers are based on diameter of the bullet. So, a .22 is smaller than a .270, a .270 is smaller than a .30-.30, a .30-.30 is smaller than a .338, and a .338 is smaller than a .577 (which, if you want a laugh, is called a T-Rex and was designed to kill large, African Game. Check this link for some people that mostly don't shoot it well. http://www.youtube.com/watch?v=wrImp-ek3bI Not that I would, that thing looks like it'd hurt). Now, the tricky thing in all this is that when you start adding extra numbers, the bullet dimensions may change. So, when we hear .223, it is the same diameter as the .22 (for the most part), but it is a longer bullet with a bigger shell casing, more powder, a longer distance over which it is accurate, and a higher power when it hits the target. Now, at the same time I say that, anything that is larger than a .223 in number (and that includes EVERY shotgun gauge) is more powerful. That part of the "Assault Weapons Ban" is tricky and messy and people don't like that reality. Want to see the difference? Check this site: http://mediamatters.org/embed/static/clips/2013/01/18/28594/fnc-hannity-20130118-bulletholes The important thing to remember is that the larger the number here, the bigger the bullet, for the most part.
Now, the reason it says "some pistol" is because it has become popular of late to allow for the .410 shotgun shell to be loaded in the same gun as .45 Long Colt (which is different from the .45 ACP due to the length of the cartridge. Bullet width is the same, but not the length of the casing). The Taurus Judge pistol started this, and other firearms manufacturers have followed along with it. These are all revolver style for the most part in pistols. Some rifles have this as well, but not as often as pistols. In pistol ammo, you will find the similar caliber process to rifle, but the points to consider are: .22 less than .38, but .38 and .357 can be discharge from the same gun (chambered in .357)...however the .357 is a much hotter round as the shell is much bigger. Then you (mostly) follow the chamberings on up in size with .40, .41, .44, .45, and yes, even .50. Now, if one added an extra 0 on the end of the .50, you get the .500, and we descend into more facts that most of you don't need to know.
Gun Function and the AR platform
Overall, we have heard a great deal of argument on gun function, how dangerous the .223 is, how many rounds one should be able to put in a magazine, etc. For all the currently legal guns, each has a purpose. NY's "7 round limit" is actually a ban on handguns, as there isn't a magazine fed handgun on the market that holds less than 8. The proposed limit to 10 rounds per mag so a shooter cannot possibly burn through 30 is also a bit flawed (http://www.youtube.com/watch?v=b2Upjn5DR0o&feature=player_embedded this video also has some great info about guns on it). Now, all that was a bit random, so let me focus in a bit. To ban a firearm like an AR because it was "designed as a weapon of war" would mean that we should also ban the M1 Garand (the basic battle rifle of WWII), any rifle chambered in anything that our current snipers use (.30-06 [one of the most common calibers in use period], .308, .300, .50), and any pistols that our servicemen use (such as the 1911, a single action, semi-automatic pistol typically chambered in .45 in service for the United States since 1911). That argument doesn't work. The AR platform has proven its use for hunting all kinds of animals, specifically varmints (like coyotes) all the way up to deer (yes, this is actually a growing in popularity caliber for deer hunters). And for the guys that hunt coyotes out West, hitting a coyote moving at 30 mph with a single shot and no semi-auto just isn't going to happen (very often). To state that the .223 is more dangerous than the large number of rounds larger than it is also false. To claim that a semi-auto can lay down fire as fast as a full auto is also a HUGE mistake (and this little mix up is a big deal if any politician makes it). The reason the AR platform is so popular, and growing more so, is the fact that it is highly customizable, easy to shoot, accurate, and for the guys coming out of the military, familiar (this is a side note, if you check the history books, there's a lot of reasons why the .30-06 is so popular. One is that the M1 mentioned above was chambered in it, and many of our GI's used this in WWII. If you check gun production after WWII, you will notice an uptick in "military style" firearms). Also, to all a semi-auto firearm an "assault weapon" is also misinformation. Is it a military style weapon? Yes. Does it function the same? Nowhere near. The military style has full auto, a 3 shot burst, and a single shot. The average, non FFL Class 3 only has 1 option: Bang. Not Bang-Bang-Bang, just Bang. That is what most people have in their closet, safe, or wherever it is they keep it.
Now, if anyone has anything to add to the information (not the opinions) of the above, please let me know. I'm sure I'll remember some random action (I did not mention muzzeloaders, but the name should speak for itself there) or some interesting point of caliber, or something else that I'll eventually edit in here somewhere, but that's what it is for now. Thoughts?