For some time now I have thought that it would be good to have a thorough, simple, educational, and comprehensive guide to K'NEX guns, anything from the particular type of gun, to the ammunition it uses. In this guide, you will find a list of the different types of K'NEX guns, magazine setups, propulsion options, internals, triggers, ammo, and them some. Without further adieu, lets move on to the first step.

Note: Main picture courtesy of SLDxRaPiiDZz - knex armory V2.0

Step 1: Gun Types

The first thing to do, in building a K'NEX gun, is you need to decide what type of gun you want. The list below is a list of all (at least I believe so) the K'NEX gun types there are, and their description, with a link to one the best versions (in my opinion) of that type of gun (some of them will be mine, no, I'm not conceded):

Bolt Action (BA): A K'NEX Bolt Action gun is a gun that uses a bolt to push rounds/bullets/ammo out of the magazine/clip into a separate chamber.

The S2 (see picture #1) is one of my favorite BA guns. See it here: https://www.instructables.com/id/The-S2-Bolt-Action-Pistol-1/. This is definitely a gun type that you should try, as it is one of the better types.
Pump Action (PA): A K'NEX PA gun uses a pump (generally on the bottom - front half of the gun) that pushes the firing pin (see a later step for details about firing pins) back, so that it can be fired again. Some guns even use both the bolt action mech (short for mechanism) and the pump action mech combined. This type of gun is/are generally the fastest firing gun.

The REMPAR-2 (see pic #2) is an amazing PA gun that you should definitely check out/build. See it here: https://www.instructables.com/id/REMPAR-2-instructions-REmovable-Mag-Pump-Action-R/
Slingshot: A slingshot K'NEX gun uses a rubber band as the launching mech. This particular mech holds a band that the user has pulled back to the chamber area, and then when the trigger is pulled, the band launches forward, pushing the ammo with it.

I am pretty sure that most hard-core K'NEXers will agree that the SRv1(see pic #3) is the best slingshot around. You will find it here: https://www.instructables.com/id/SR-v1/.

Break Action (BkA): A BkA gun uses the middle of the gun as the "loading bay." When you "break" the gun open, you can insert one (sometimes two)rounds, and then fire.

Seleziona's Break Action Rifle (see pic #4) is one that I particularly like. Find it here: https://www.instructables.com/id/Selezionas-Break-Action-Rifle/
Simple Pin: A simple pin gun does not have anything special about it. With these guns, all you have to do is pull back the pin and pull the trigger. These guns are my particular favorite, and are mostly found in models/replicas (K'NEX versions of real guns).

On of my favorite simple pin guns is Senior Waffleman's SCAR-L (see pic #5). Check it out: https://www.instructables.com/id/Knex-Scar-L-1/
Lever Action (LA): One of the most uncommon guns, the lever action gun is relatively hard to describe, as I have never built one, and am not sure how they work, and I have never seen a really good working one.
Bullpup: A bullpup gun has the mechanism and mag BEHIND the handle. This means that ALL the shooting action is behind the handle/grip.

My favorite bullpup of all times is Sharir1701's M17S (see pic #6): https://www.instructables.com/id/bushmaster-m17s/
Plunger: Essentially, the plunger pushes the rounds out of the barrel by being pushed by some other force. See the video below to understand more fully:

Turreted guns (TR): These guns generally get the best range, as they separate out each bullet, and give users the chance to fin (add fins too) their ammo. The gun has a turret, that allows you to push bullets into 4, 8, 12, or 18 different "mini-barrels."

See KILLERK's TR18 (see pic #7): https://www.instructables.com/id/Trifle/
Breach Loaded (BL): Breach loaded guns are, by far, my favorite type of special K'NEX (non-simple pin gun). They have a breach that is generally on the side of the gun, wherein you put the bullets. These guns don't really allow you to used fined ammo, but they get good range all the same.

My recently posted RBLTR (see pic #8) got good range, and I think you guys should give it a look (if you have not already). You will find it at this address: https://www.instructables.com/id/KNEX-RBLTR-V1-Reds-Breach-Loading-Tactical-Rif/
Semi Automatic (SA): SA guns have the ability to shoot EVERY time you pull the trigger. This means that you don't need any firing pins or slingshot rubber bands. This type is by far the most uncommon gun to find, as they are a difficult build (generally).

Here are addresses to some of the better semi automatic K'NEX guns: http://knexinnovation.net/showthread.php?1108-WASP-War-Assault-Semi-automatic-Pistol, https://www.instructables.com/id/the-S3-semi-automatic-gun-SA-3252/, https://www.instructables.com/id/The-Kinno-2-A-Semi-Automatic-Knex-Gun/, and the https://www.instructables.com/id/The-Semi-Auto-Blowback-Pistol-Instructions/
Hammer gun: As is seen by the name, hammer guns are operated by a hammer which is generally pulled by rubber bands. These guns are relatively obsolete, but are cool none the less.

Here is a link to one I recently built: https://www.instructables.com/id/Knex-Pistol-with-a-NEW-firing-mechanism-a-Hamme/
Not A Rectangle (NAR): The NAR was posted on K'nex Innovation on October 2007, and it revolutionized (or so I am told) K'NEX guns. I will let the actual gun and the actual poster (oodalumps) speak for them selves.

Here is a link to the first NAR: http://knexinnovation.net/showthread.php?812-Not-A-Rectangle

Lets move on to step two, as that will give you magazine choices.

Step 2: Magazines

Now that you have a K'NEX gun in mind you kinda need something that is gonna hold your ammo, and thus, you need to decide what type of magazine/bullet-sitting-area you want to use.

5.56 Magazine: These mags were modeled after the STANAG magazines of the 20th century. The are reliable and easy to make and use.

See here on how to make (see pic #1): https://www.instructables.com/id/G36K-V3/step3/Magazine-and-Stock/
7.62 Magazine: Modeled after the Nato 7.62 magazine, this one is quite popular.

gtrain was one of the first to build/use this type of mag (see pic #2): https://www.instructables.com/id/Knex-M21-EBR/
9mm Magazine: Modeled after the actual 9mm magazine, this one generally shoots dark grey, one slot connecters.

See a version here: https://www.instructables.com/community/Knex-MP5/
Tubular Magazine: This type of magazine is more generally called a horizontal magazine. What a tubular magazine is is rather hard to describe, so let me give you a link, so you can view it for yourself.

My RTMP (see pic #3) has a nice, fully functioning tubular/horizontal magazine. See it here: https://www.instructables.com/id/RTMP-Reds-Tubular-Magazine-Pistol/
Hopper Magazine: A hopper magazine is probably one of my favorite types of magazines. The allow you to make them as big as you want, and generally hold a large amount of bullets.

My Calico M-950 has a full instructable on how to build its mag, check it out: https://www.instructables.com/id/KNEX-Calico-M-950-Build/
Bolt Fed: (see "Bolt Action" in step one)
Muzzle Fed: When you muzzle fed a bullet, you are either putting/dropping/or placing a round in the barrel, which is then fired out again. No magazine type needed.
Pre-Loadable Magazines: A pre-loadable magazine is a magazine that you can load and band BEFORE you put the magazine in the mag well (the magazine "hole"). These types of magazines are kinda hard to build, but they are worth it.

Here are links to three great pre-loadable magazines instructions (you might have to look through full 'ibls to find them): https://www.instructables.com/id/the-S4-a-new-type-of-bolt-action/, https://www.instructables.com/id/Knex-Removable-Magazine-New-Mechanism/, and https://www.instructables.com/id/Improved-Removable-Mag/
Pistol Grip Magazines (PGM): PGMs are a small magazine that goes up the handle of your pistol. Blue Mullet has really made this type of gun big, and a ton of people have made derivatives of his version (see pic #4).

See his great gun here: https://www.instructables.com/id/Removable-Grip-Magazine-Pistol/

Step 3: Propulsion

Now you have a gun and a magazine... but you need something to help you use them. Propulsion. There are two basic types of firing types/propulsion. Pins (pic #1) and rubber bands (pic #2). A rubber band is not used as often as the pin is, as the pin mech is generally easy to use, in collaboration with the rest of the gun you are building. A pin can be built of any rod size (preferably the light grey rod/black rod/orange rod/clear rod. Any size rubber band CAN be use to launch bullets with, but #117 or #64 size rubber bands are generally the best. Picture #1 is of a firing pin of the largest size, it has electrical tape and a broken band to keep it sturdy and strong. The former for strength, the latter for softening the impact of the pin hitting the gun. The 2nd picture is of #117 size rubber bands. These are ones that I like to use for slingshot guns, as they are long, and easy to pull.

Step 4: Internals and Triggers

So, now you have a gun in mind, a magazine that you want to use, you have decided weather to make it a slingshot-like-gun or a pin gun, but you are missing something. That something is internals. Internals are what operate your firing mechanics, which in turn operate your gun.

Now, there are thousands and thousands of different types of internals, and I could not possibly name all of them for you, so I will name the three best and simplest that I can find.

In the first picture you will find a mech that I like to call the "Dr. Richtofen mech." The reason I call it that is because it was designed by dr. richtofen, and is sturdy and reliable. This one would be easiest for newer K'NEXers. Click on the picture to learn more about it. Here is how to build the gun this mech was used in: https://www.instructables.com/id/knex-pistol-34/

In the second picture you will find a mech that was designed by gtrain. Click on the picture to see how it works. Find the gun it was used in here: https://www.instructables.com/id/Knex-G36C-instructions/

In the last picture of a mech designed by Senior Waffleman, and was used in his SCAR-L. It is one of my favorites, and can be used in many guns. Again, click on the picture to see how it works. Here is a link to the gun it was used in: https://www.instructables.com/id/Knex-Scar-L-1/

Trigger(s):

Aside from the triggers you see below, there is one type of trigger that is most commonly used. It is known as the "True Trigger" (see pic #4). I am not entirely sure who was the first person to build a trigger like this one, but I believe it is the best ever designed. It works on a pivot, and can be made and used many different ways. Click on the picture to get a better understanding of how it works.

Step 5: Ammo

So now you have your gun, your mag, your propulsion system, your internals, but you need something to fire. Ammunition (ammo).

This is by far the most controversial topic in the K'NEX community, as everybody has a different opinion on ammo. I really don't like rod ammo, and I really don't like connector ammo. I feel that a mix of both is the best, but there are many people who disagree with me.

There are four basic types of ammo: rods, connectors, wheels, and a mix of rods and connectors.

Rods (pic #2):
Rods are by far the most used K'NEX ammo. They are easy to use and generally get good range. The two best types of rods to use are yellow and blue, as these get the best range. Anything bigger than these two types create too much drag, and don't get very good range. Any type smaller than a yellow or blue rod don't seem to work very well, and I am not exactly sure why that is.
Connectors (see pic #3):
I would rather use connectors than rods, as they always work better for me, but it is all really based upon opinion. The picture I have shows all the connector types, but the dark grey connector, the orange connector, and the small tan connector (not shown) are the ones that are really used for guns.
Wheels (see pic #4):
This type of ammunition is rarely used, as they require a somewhat large gun. If you take of the tires though, they make a great grenade type ammo. Their circular shape makes them easy to load and shoot, and they generally get good range (sometimes 100 ft. plus).
Mixed type (see pics #5 - #6):
The most common mixed type of ammo is known as oodammo (see pic #5). It gets good range because of it arrow-dynamic shape, and because of its size. I prefer mixed ammo (generally a dark grey connector attached to a little green rod), as it preforms best for me.