I'm probably the last person to criticize, so I can give You some advices based on my opinion how to make pixel things look better.
I think this crossbow is a bit uncomfortable to hold. Maybe try to smooth the corners to make it look more realistic. If You wanted to make it very blocky, You did it, but I think this plank on the front should be curved backwards to put a string on it. And it should be thinner, because now it'd pull the front of weapon down and add a lot of weight to construction. You could add more details in different colours e.g. metal mechanisms, bone nails, some paint, or something like this.