Want to make your own barrel attachments for your rival Challenger? This is where to start.

This model has the lugs that attach to the muzzle. Tested to work!

The height of the template barrel is enough to cover the standard lug. Just extend it as long as you want, put some speed holes in it, make the outer diameter thinner or thicker, your imagination is the limit!

Note: The images show a longer template. It is shorter and stops right at the edge of the muzzle.

It works with my Rival barrel muzzle model: https://www.printables.com/model/1136566-nerf-rival-muzzle-3d-reproduction 

This model will also fit the Sillybutts Battle Axe Rival Muzzle: https://www.printables.com/model/1135814-rival-barrel-mount-muzzle-for-sillybutts-battle-ax