WARNING!
THIS IS NOT A SIMPLE MODIFICATION PROCESS!
https://flechettespectre.blogspot.sg/2017/03/custom-made-systems-for-nerf-crossbolt.html
https://www.youtube.com/watch?v=ugythr184fw
there is a lot of shell cutting and test required for this mod to be done.
you will require various stuff,
such as
2mm thick wires, about 30cm long to bend and make a pusher.
90-degree toy gearbox