A crossbow is a bow mounted on a stock. The bow fires a small dart called a bolt or quarrel. Crossbows were common weapons in medieval times. More powerful than regular bows, they could more easily pierce armour at greater ranges. Crossbows took a long time to crank and shoot. Slower than standard bows, they were eventually superseded by firearms.
Begin with two pieces of wood. These should be thin pieces of equal width with one piece 3 feet long and the other 2 feet long. The longer piece will form the stock of the crossbow. Essentially the stock is similar to that found on a gun, except there is no barrel. Take the shorter piece and attached it crosswise to the bottom side of the longer piece. Place it near one of the ends of the longer piece. You should have something that looks like a "T" with part of the "T" sticking out above the crosspiece.
Drive two small nails into the top side of the crosspiece near each end. The two nails should be equal distances from the stock, or longer piece of wood. You can use wood screws instead of nails. Allow the nails or wood screws to stand up slightly from the crosspiece.
Obtain an elastic band or wire. Tie one end of this band or wire to one of the nails. Take the free end of the band and pull it back toward the far end of the stock. A few inches from the end of the stock place your finger on the band and hold it down. With your other hand, take the free end of the band and pull it towards the other nail. Wrap the end of the band around the nail. This will be your bowstring.
Lift your finger from the middle of the bowstring. The bowstring should snap back toward the crosspiece and lie evenly on top of it. If it does no, redo it. If the bowstring is aligned properly, tie the wrapped end securely around the other nail and cut off any excess. Use a stick or small metal rod to pull back the bow. Place a projectile against the taught bow and release the stick to fire your crossbow.
To make a more powerful crossbow, use a flexible lathe or thin flexible metal rod in place of the stiff crosspiece. The new crosspiece must be strong enough to withstand the pressure of the bowstring. The bow will spring back when the crossbow is fired. The spring action will add additional power to the shot.
To make an extra powerful crossbow you can install a cranking mechanism to crank your bowstring back to firing position. You must also use an especially sturdy and flexible bowstring.
Be careful when firing. Do not point a loaded crossbow at anyone.