As a warning I must warn you this mod involves mutilating your gun. I was hesitant at first but then I just kept telling myself "It's just the barrel extension" so I was able to stay sane (to a degree) during the mod.
First, what we want to do is take the gun apart, and take out everything on the top half of the gun, which is pretty much just the Longshot barrel's extension. It should look like this.

Now, put the whole thing back together (this was a really tedious process for me and my fingers hurt from all the screwing).
This is where I started questioning my sanity. Take a knife or saw, and cut off the top half of the gun. It should look like this.

After that, put some tape on the open spots at the top of the gun to protect the insides. You can repaint the gun to make it look cooler, but I don't paint my guns, so yeah. Hah, it looks more like a water gun than a Nerf gun now. Enjoy your new pistol!