How-to's that can be used with almost any rubber bands or rubber band guns!

Step 1: "Spur Gear" Rubber Band Guns

If you have a rubber band gun with a "spur gear" in the back (the most common kind of modern rubber band gun), then you're in luck: I have a modification!
MODIFICATION:
If you want to modify a rubber band gun with a spur gear, use whatever is sturdy and on hand! I chose to make mine out of cheap but thick particle board(?), as it is the easiest for me to work with, and yet pretty sturdy (if worked with just the right way). First things first: decide what kind of weapon you want. There are a few basic/general types:
-Pistol
-Shotgun
-Machine Gun/Submachine Gun
-Rifle/Sniper Rifle
I'll start with the Pistol: If you have something that ISN'T a pistol, just shorten it a lot!!! It's as easy as pie. Shorten the "barrel", and make a notch at the end. Add whatever is needed if you have a pistol (lengthen it).
If you want a Shotgun, then take out a few pieces of wood. Make a wide "barrel". If you want, make a "barrel" tip like mine, as it allows for more rubber bands than any store-bought rubber band gun ever will with a single "barrel"!
Machine Guns and Submachine Guns are pretty easy, as there plenty of designs. Mine is based somewhere between the Uzi and the MP-7 and the machine guns that were used in The Incredibles, by Syndrome's guards. Those things were pretty good. I have yet to make a scope, though...
Rifles are the second-easiest, besides pistols, as you just lengthen the "barrel". Really easy.
FOR ALL OF THE GUNS:
Make a notch of some sort at the end of the "barrel".
ANOTHER MOD FOR ANY GUN:
But, then again, you might just want to make it look cooler. If you want that, either add a "silencer" or start looking for features of guns you think look cool. A "silencer" is just a piece of PVC pipe that fits loosely (or very loosely) over the "barrel". Sadly, the name is misleading, as the gun makes even more noise than before. Beware, the "silencer" may affect your rubber band gun in unexpected and unwanted ways, such as spreading the rubber bands and throwing off accuracy slightly. If you do this mod, then never fear: you can always take it off! It is held to the gun by the tension of the rubber bands. This can let you do crazy things, like firing 3 shots with the "silencer" on, taking off the silencer, and revealing 3 shots not using the "silencer".
SPECIAL NOTE ABOUT ANY GUN:
If you have the small bar as seen on my Compact Rifle-1or pistol (stock), DO NOT remove it, as it keeps the rubber bands from jamming the gun, and out of the firing mechanism. If you don't have one, never fear! Just bolt, nail, screw, glue, or...fuse?...any prism with a flat surface on one side (or not, if you're not gluing or fusing it on). The prism should be anywhere between 1 inch and 2 inches in front of the trigger mechanism, but keep in mind that prisms with rounded tops keep the stress off of rubber band guns. Besides, I like them better.

Step 2: For Those of You Who STILL Use Band-It Type Rubber Band Guns.

If you have a rubber band gun that uses a simple clothespin thumb trigger, look here!

Like the Spur Gear rubber band guns, Band-It guns can also be made into pistols, shotguns, rifles, and machine guns. Except machine guns must either have a lot of clothespins or can't have much ammo loaded.
Pistols are the easiest! Just take a piece of wood, make a 45-degree angle cut at one point, and glue the pieces together in any way you want! Then just glue a clothespin to the top, around the barrel-handle joint, and make a notch at the front.
Machine guns are just wooden models of the real thing, except with a notch at the end of the "barrel" and clothespins on the top, near the handle.
Rifles are generally just pistols with LONG barrels (and possibly scopes or sights).
Shotguns can be the hardest, as you may want to fire it with one "barrel", two "barrels", or the two types of "barrels" combined! In the last case, just copy my design, minus the TINY notches, as rubber bands can get stuck in them REALLY easily. Just make the rounded-off notches, the widest ones. For this, take out either your rotary saw, your hand file/sanding-type thing, or even a Dremel with some sort of high-strength cutting wheel or other thing...and cut the rounded notches already!!!

If you want to make either the revolver or shotgun, here are a few hints:
-The Revolver requires a peg of some sort that won't come loose no matter how much you turn it. NEVER fasten the barrel to the handle permanently, as it must rotate (or revolve). For best results (or not), make a bunch of Band-It pistols on the sides of a prism of some sort, whether it's hexagonal, octagonal, or square. Equal and even shapes work best (hexagons, octagons, squares, etc.), but are countered (undone) if you don't drill the hole IN THE MIDDLE!!! The whole assembly will be the barrel, by the way.
-The Revolver needs slits or slots or wide grooves of some sort to make sure you don't have to twist rubber bands to load them, which will be the downfall of your rubber band revolver if you're not careful.
-The Shotgun can fire using two barrel ends: the regular, commonly used pistol- or rifle-style basic horizontal slot (be careful about firing order to prevent jamming), and the plus-shaped slot. The advantage of the plus-shaped slot is that you can fire in whichever order you like, if you figure it out. Another way to use the plus-shaped barrel is to have two separate barrels. This will be a lot easier (just make a notch in each of them), but needs more material and may look nicer or more authentic.
-The Spreadgun is like a shotgun on steroids, but it cannot use rubber "buckshot". It requires two slots per barrel and three barrels (but yours will probably have different amounts). Three barrels, three rubber bands per barrel, nine shots total.
Don't want to download the video? It's on Youtube:

Step 3: Hand Shooting

As the Guide to rubber band shooting with hands is dead, I'll just jot down what I know. Comments on this page will be considered part of this page, because I don't know too much about hand shooting.

1. Classic/Shotgun: Stretch the rubber band using your right index finger and right thumb from your left thumb, and let go with your right fingers to fire. Not the easiest or most accurate, but very quick.
2. Pistol: Hold your right hand in a fist in front of you, then raise your thumb and stick out your index finger and pinkie. Put one end of the rubber band around your index finger, around your thumb, and onto the tip of your pinkie. Then press your pinkie to your palm to hold the rubber band, and move it away from your palm for a second to release the rubber band. Easy when practiced, somewhat quick and easy to aim.