This model is a friction-fit barrel insert for the Nerf Rival Knockout that allows for firing MegaXL darts. No mods to the blaster are required. Just insert into the barrel until it snugs the hop-up (or until you can't go any further). Easy to insert and remove. 

With a stock Knockout blaster, expect an average of ~50 fps muzzle velocity. I've measured a range from 45 fps to 55 fps. A stronger spring will yield higher FPS, of course.

This will also work with the Rival Pilot blaster, but it doesn't seal as well and is more finicky. But it does result in about 35-40 fps muzzle velocity in the Pilot. It's just enough to be useful but not nearly as good as the performance of the Knockout. 

Printing Notes

Print with the small diameter end down on the plate to avoid using supports. Or you can print it with the larger diameter down and add supports. Use a brim if you have adhesion issues or just as a precaution. I use 6 outlines with 2 layers and 0 offset for added print stability. Peels right off when the print is done. I recommend 3 perimeters all around and 15% infill, but feel free to experiment. None of those parameters are particularly critical for a functional print.