Step 1: "Spur Gear" Rubber Band Guns
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:
-Machine Gun/Submachine Gun
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.