Optimize crossbow I'm currently building a crossbow 

and was wondering how I might improve the performance of it? I was suggested to fine-tune the rubber band more and maybe change the projectile maybe to a zinc alloy one instead of the plastic ones I use. I do understand this is sort of engineering feat but I think it wouldn't hurt to hear feedback from some physicists so I would appreciate any insight into this. 
 A: "Performance" is a pretty broad term.  For example, it can relate to how fast the bolt (the arrow) is launched, or the repeatability of the bolt's trajectory.
Plastic bolts are not a very good idea: most plastic can deform permanently, which will lead to a wildly variable trajectory.  You might do well to buy some fiberglass or graphite fiber fishing pole blanks and cut the ends off to make your bolts.  Light weight is good.
The length of the bolt is important: if it is too long, it will flex while being launched, which can result in an unpredictable trajectory.
Using rubber bands you've made more of a slingshot than a crossbow.  Take a look at the design of a recurved bow or a Mongolian bow if you want a really powerful crossbow.
A: A better crossbow design than a fixed wooden rod and an elastic band is a tough piece of string connecting to "sprung" arms that are pivoted at the join. They can be sprung by winding up elastic as the string is pulled back.
This makes for a better crossbow as it is much more effective at storing elastic potential energy as the mass of the arms move as well as the projectile.
This idea was used in Greek catapult's such as the palintonon:

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