Semi-automatic Crossbow

Auto-loading crossbow with a 4-round magazine
Late 2018

The goal is to maximize the shooting efficiency by coming up with a auto-loading mechanism.

Prologue

After I got into Berkeley in August 2018, I looked for ways to make use of the makerspace and practice my skills in designing more complex mechanisms. The project that I decided is a crossbow that can load arrows automatically. This project serves only as a practice.

Design

Close rail design

Most commercial crossbows have one side of the rail exposing to the outside. However, if there is any perturbation along the rail that causes the arrow to rotate, the string force then generates a moment that may cause the arrow to enter an unsteady state. As a result, I decided to close the rail completely by covering the other side.

Auto-loading mechanism

To achieve the goal of auto-loading, I have been thinking of the mechanism to use. Since the arrow must be load after the string has been pulled back, it is most desirable to use the string to trigger something to load when it is pulled all the way back. As a result, I designed this mechanism in which the string triggers the arrow to unlock.

The magazine

The magazine in a gun uses rectangular spring for loading, however, such springs are really difficult to find. As a result, I applied the loading design of some old rifles to my magazine. The elliptical shape thing is to ensure that the pushing bar is always pushing at the center of the loading block.

The bolt

In order to load, the bolt must be opened. Therefore, I designed a bolt that can be easily unlocked to load and cut out an opening on the structural plate for loading.

The limb

Since the crossbow limbs are too expensive on Amazon, I decided to use a rubber band instead. The most desirable limb structure to balance the tension is obviously the triangular beam. However, since it produces interference with the hand of the shooter when loading, I have modified it to a not interfering beam.

Undesirable arrow pop up

After the first shooting test, I found that the arrow may pop up after one shot is launched. The reason for that is the arrow locks may be opened by the friction from the just-launched arrow when it travels along the rail.

Lock up the locks

To solve this problem, I have changed the shape of the trigger to make it interfere with the push bar to stop it from moving. Since push bar cannot move, the arrows locks that are connected to it cannot move and it is then not possible for the arrow to pop up undesirably.