Print the trigger at 100% infill to make sure it's strong enough for many shots. Use the stock internals of the Nerf Jolt, you can take them out by removing the trigger pin and unscrewing the 4 screws at the bottom.

A different version of the Nerf Jolt meant to look more realistic and detailed than the original version. This replaces the main body and trigger mechanism of the standard toy and the only modification required to the spring assembly is to cut off one of the small plastic tabs (I will add a picture).

The insert is not required and I just put it in to replicate the standard gun. If the barrel is too snug you can sand the top part of it a bit and ammo should fit fine. If you have any questions let me know and I will try to answer them.

I added the blender file to make it a bit easier to modify