Since you guys asked me so many times for this, here it is. I have to warn you- if you dont make these parts PERFECTLY, they wont work worth a damn, so you'd be better off buying them in a store on on ebay (like I do).
I would always use some ABS plastic sheet for this, but if you find yourself short on this material, use very strong hardwood (rosewood, oak, maple - hell, ironwood if you can get that [good f***ing luck]). You should plane the material down to 1/4" for best use.
Step 1: COG Wheel: Sketch
First, using a specialty drafting ruler or compass (and anything with 8-12 angles marked on it). Use the ruler and sketch the circle (around 1" in diameter), and the tips of the angles. (as shown)
Step 2: COG Wheel: Sketch Outer Circle
Now, using a compass and ruler, sketch another circle (on the same centre point), one inch out from the edge of the first circle.
Step 3: Cog Wheel: Arm Definition
Now, on both sides of the 8 (or 12) of your initially drawn lines, measure about 1/8 of an inch from each side, so that you have drawn a 1/4" section on each of the lines, creating the arms. (as shown below)
Step 4: Cog Wheel: Trimming Your Lines
Now, erase the lines you have drawn so the diagram on your piece of hardwood (or abs plastic) looks like this:
Step 5: Cog Wheel: Drill the Axle Hole
Now, using a 1/4" drill bit, bore out the axle hole in the centre of the cog wheel, using your very first lines to mark the centre.
Step 6: Cog Wheel: Cut It Out
Now, using any tool you want (I highly reccomend a well-calibrated band saw), cut out the cog wheel.
Note: if you want, add little bumps at the top of the arms on one side consistantly around the wheel to help keep the bands from slowly slipping off if you have the gun loaded.
Step 7: Trigger: Draw Mechanical Profile
Now, with your newly finished cog wheel, place it on another piece of hardwood (or abs plastic), and draw the profile of the mechanics of the profile.
The top of this sketch should show a shelf for the top of one of the arms to sit in. Then, draw a broad sloped line with PLENTY of room for the cog wheel to move in (noted), then the bottom of the sketch should show a shelf well below the bottom arm it intersects, but it should be protruding into the arms' way when the trigger is pulled (explained later)
Step 8: Trigger: Finish the Sketch
Now, finish off the trigger by adding the finger slope (commonly known as the trigger), then allow about 1/2" from the slope previously drawn. When you get near the top, draw above the shelf and come down upon it.
Then, draw places to drill 1/4" holes for the axle (in the middle), and the trigger block (near the top shelf)- the trigger block keeps the trigger from being pulled back into the cog's mechanism and blocking it.
After the holes are marked. drill them out.
Step 9: Trigger: Insert the Trigger Stop
Now, insert a 1/4" dowel into the top hole you drilled for the trigger stop. it should protrude about 1/4"-1/2" from either side of the trigger.
Step 10: Mark Them for Use!
Now, referring to my other instructable, High quality RBG's, follow the steps for using the parts you just made- hopefully you made them well!