Threw this together quick. Its untested but it won't break your game if missed something. It doesn't add any new models or anything, just a quick edit of the regular crossbow where I changed the magazine size to 8. You can change it to whatever you want. Just search for MagazineSize in the items.xml. Added a recipe file so it can be crafted as well. Upped the required resources a little bit from the regular crossbow but you can change it however you would like.
Edit:Changed the burstround count and upped the rounds per minute a bit so it should autofire now. Numbers may need a little adjusting.