Step 1. Build the bow and prepare the string
Prepare ¼” poplar strip with ¾” width and 12” length. Cut the strip to length as to make the bow. Soak the poplar strip into water all night. Then, clamp the strip into a concrete round surface. Leave it all night to hold its curve shape.
Drill three holes sized 1/8” into the strip; one drill hole is in the middle of the strip, and one drill hole on each side with ¼” from every end. The string for the bow is like a tiny bungee cord with 10” long. Buy the strings at the local supply store.
Step 2. Decide the length of the draw
Attach the bow into 1×2 pine end with screw and glue. On each side of the bow, install the string hook. Place a finger onto each side of the pine and pull the string back as far-off as it can go. Give marking on the distance with pencil.
Step 3. Build the notch and the firing pin
From the pencil mark, make a deep cut as the string depth. Make one other cut with 45 degree angle from the same cut for the notch. There will be two parts; the flat and the angle parts. The flat side should face the bow while the angle side should face the back. Use sandpaper and a file to round the flat front corners to release the string easily.
Drill one hole at notch center all through the lumber. Insert ¼” dowel into the hole for the firing pin. Draw out the string up to the notch and depress the dowel. Then, push the dowel from the bottom up so it releases the string.
Step 4. Build the retainer and the trigger
To be able to shoot the bottle caps, the crossbow pistol needs to have the retainer and the trigger. The retainer is used to load and grip the bottle cap until it is ready to be fired. Prepare some large sticks and cut them using shears.
Cut one stick at 2/3 part as for the retainer to hold the cap steadily. Cut three sticks as shown in the picture. Pile and glue them altogether. As for the trigger, use two sticks at their full sizes, pile and glue them altogether.
Step 5. Finish the assembly
Attach the retainer onto the pistol by gluing and screwing it as shown in the picture. Make a cut at the angle in front of the firing pin on top of the bottom side as for the trigger. Make necessary adjustment on the firing pin length. Then, attach the trigger into the angle. Secure the joint using screw and glue. You may also add your own grip design. Test the pistol with some bottle caps to see if it works well.