Crossbow design
https://www.instructables.com/How-to-Make-a-Repeating-Crossbow/

We want to replicate this "chu ko nu" crossbow and enhance it with the automatic reloading system we will design.

-A 2x4 in any type of wood, prefer few defects of course (at least 27" long)
-1/2" Sch 40 PVC pipe (at least 22" long)
-Red oak 1x2 (53" or more, other hardwoods are OK)
-A 1/2" x 5.5" x 48" poplar board
-3/8" poplar dowels (recommend three 48" dowels)
-5/16" dowel (at least 10")
-Larger dowel, 3/4" to 1" diameter (at least 6" long)
-Glue
    -white glue
    -epoxy
    -hot glue stick
-String (I used 9/16" venetian blind chord, it is mixed polymers)
-Two 1/4-20 bolts, 3" long, with nuts.
-Wood screws (at least 6, ones that are 2"-3" long would be great)

OPTIONAL:
-Wood stain
-Spray on clear coat
-Brown spray paint
-1.5" wide hinge
-1.5" wide by 1/4" thick oak or poplar board (10")

https://maker.pro/custom/tutorial/what-type-of-motor-is-best-for-my-project

We want to use 1-4 brushless DC or stepper motor to pull on a drive wheel which will spin and pull a conveyor belt around the main shaft of the crossbow. There will be a hook attached to the conveyor belt which will hook on the crossbow string and pull it as it moves along the shaft. as it goes back, it will let go of the string as it goes around the shaft again.