The pistol bow is something I made with scrap from other projects, but the pieces were all pretty common or easily produced shapes, so I figured I would submit this as a concept for inspiration; it is not an exact guide.
Note: this is a device that can fire projectiles, do not aim at any living thing when it is primed or loaded.
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Step 1: Gathering Parts
As I mentioned, I used scraps. However, most of these pieces are common, or cheap, enough to gather with ease. Because I do not know the specs of any of these parts, I will not provide definitive measures, just generalities.
~1/2" wide metal C-channel. About 6" long.
2 pulleys, small. Mine are ~1" around, they are great.
Wooden ruler, plastic is too brittle.
Small handle piece. I made mine out of two small wooden planks bolted together, you can just use something that you can drill holes in, about the same width as your C-channel, and is easy to hold.
Hardware, excluding the handle: 3 bolts and 7 nuts, about 5mm in diameter.
A few inches of 1/2" PVC or CPVC pipe.
Rivets, could use bolts but rivets are lower profile where I used them.
Thin wood stock or sheet. Mine is ~1/8" thick.
A couple rubber bands.
(Not all materials are shown on paper, some are built into bow: C-channel, ruler and handle piece.)
Step 2: Assembling Cross.
I drilled two small holes in the ruler and C-channel. It is extremely important the two pieces are squared up, so I drilled one hole in each, riveted, and used that as a hinge to keep both pieces steady for the next hole and rivet.
Note that I attached the pieces at the 6" mark, the center, on the ruler.
Step 3: Attaching Pulleys
I drilled holes in the ruler at the 3" and 9" marks. Screwing the hardware up through the ruler I added spacers the lift the pulleys off the ruler. This will keep the band from rubbing on the channel. For my hardware I had to use two nuts for each pulley as spacers, with a third to secure the pulleys in place.
Step 4: Fashioning the Trigger Pieces
I find it hard to communicate the shape of the trigger-hold, but I will try to illustrate it with pictures.
Drill a hole larger than the third bolt's threading straight through a 1/2" long piece of 1/2" pipe. Perpendicular to these two holes, at either end of the pipe, cut slots wider than the trigger catch piece. One slot will be on top of the pipe's "north" end, the other will be at the bottom of the "south" end. Neither will cut more than halfway along the length of the piece.
The trigger-catch is cut of the wood stock, about 2" long with a hole big enough to fit snug on the third bolt. Use any tool you are comfortable using to fashion the wood into the shape pictured above. I used a scroll saw to get the shape and a Dremel to bevel the edges and sand in the rounded shape.
Step 5: Assembling the Trigger Pieces
First, insert the trigger catch, the wooden piece, into the trigger hold. Refer to the image to see how it lines up.
Next, insert the bolt through the holes in the trigger hold, make sure to thread it through the trigger catch. It will act as an axle about which the trigger catch will rotate.
Secure with the last nut.
Step 6: Mounting the Handle and Trigger Machinism
Drill 2 holes at the top of your handle where you want to attach it to the C-channel. If you have a solid piece for you handle, you can bolt or rivet the pieces together and only drill one hole. Do not hot glue, the point takes too much abuse.
Through one of the holes, if you did not bolt the handle to the channel, thread a zip tie and secure it around the handle and channel. Make it as tight as possible.
Next, place your trigger mechanism on top of the channel and secure it with a zip tie that encircles the handle, channel and trigger mechanism. Place the bolt of the trigger mechanism behind the zip tie so that when the cable or rubber band pulls the catch of the trigger, the mechanism is held in place by the tie. Reference the images above.
Step 7: Adding the Band, Making Ammo
Hook the rubber band, or two, between the channel and ruler, then thread it over each pulley from front to back. Refer to the image.
For bolts, I used Nerf darts with a band of 1/2" CPVC over the front and back to reduce friction over the channel.
Step 8: Shooting and Finishing.
To fire, simply hook the rubber band under the trigger catch, put a bolt in the channel then pull the back of the trigger catch down with your thumb as though you were cocking a revolver. Don't shoot at anything living.
Improvements I have planned that you can do on your own; reduce the size of the ruler, find a more powerful tension source than a rubber band, construct a more sophisticate trigger and give it a sweet paint job.