This is my favorite NERF gun combo, as it's the most practical and the one into which I've invested the most time. The idea was that you wouldn't ever have to adjust the grip of either of your hands in order to prime or fire the secondary gun. Most NERF combos like this over-complicate things and mount the secondary gun upside-down, meaning you have to constantly readjust your grip to prime and fire the gun, or slam-fire it, which is never recommended. Additionally, these kind of mods require that you cut some of the gun off for it to fit, usually part of the handle, meaning the secondary gun is no longer practically functional on its own. With the setup I implemented, it is not only extremely easy to use as a combo, but the secondary gun also retains all independent functionality.
It's well-known in the NERF modding community that barrel extensions such as the one used here slightly impede dart flight, so I wanted to do something to remedy that. What I ended up doing was cutting off the front, removing the internal pieces, and then literally peeling off the parts of the barrel that would still be in the way of the dart flying freely through the barrel extension with some needle-nose pliers. The result was crude, but effective. (Here's a link to a page that shows, about a quarter of the way down, the original state of the inside of the Retaliator barrel.)
Original Voltage-Modified Version